fix: Invalid attempt to access database with closed connection

[rapsealk]

This PR is a follow-up of #835

This PR moves a database accessing code to the valid context manager scope.

2023-10-24 17:51:54.064 DEBUG ai.backend.manager.plugin.error_monitor [82177] collected an error log [ERROR] "sqlalchemy.exc.ResourceClosedError: This Connection is closed" from manager
2023-10-24 17:51:54.064 ERROR ai.backend.manager.server [82177] Uncaught exception in HTTP request handlers ResourceClosedError('This Connection is closed')
Traceback (most recent call last):
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/server.py", line 232, in exception_middleware
    resp = await handler(request)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/server.py", line 219, in api_middleware
    resp = await _handler(request)
      ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/api/auth.py", line 574, in auth_middleware
    return await handler(request)
      ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/aiotools/func.py", line 23, in wrapped
    return await coro(*args, *cargs, **kwargs, **ckwargs)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/api/ratelimit.py", line 73, in rlim_middleware
    response = await handler(request)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/aiohttp/web_middlewares.py", line 117, in impl
    return await handler(request)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/api/auth.py", line 581, in wrapped
    return await handler(request, *args, **kwargs)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/api/manager.py", line 63, in wrapped
    return await handler(request, *args, **kwargs)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/api/vfolder.py", line 2313, in delete_by_name
    await _delete(
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/api/vfolder.py", line 2227, in _delete
    await ensure_host_permission_allowed(
      ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/models/vfolder.py", line 835, in ensure_host_permission_allowed
    allowed_hosts = await filter_host_allowed_permission(
      ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/models/vfolder.py", line 858, in filter_host_allowed_permission
    allowed_hosts_by_user = await get_allowed_vfolder_hosts_by_user(
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/src/ai/backend/manager/models/vfolder.py", line 564, in get_allowed_vfolder_hosts_by_user
    if values := await conn.scalar(query):
      ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/sqlalchemy/ext/asyncio/engine.py", line 478, in scalar
    result = await self.execute(statement, parameters, execution_options)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/sqlalchemy/ext/asyncio/engine.py", line 453, in execute
    result = await greenlet_spawn(
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/sqlalchemy/util/_concurrency_py3k.py", line 115, in greenlet_spawn
    result = context.switch(*args, **kwargs)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 1710, in _execute_20
    return meth(self, args_10style, kwargs_10style, execution_options)
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/sqlalchemy/sql/elements.py", line 334, in _execute_on_connection
    return connection._execute_clauseelement(
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 1577, in _execute_clauseelement
    ret = self._execute_context(
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 1808, in _execute_context
    conn = self._revalidate_connection()
    ^^^^^^^^^^^^^^^^^
  File "/Users/rapsealk/Desktop/git/backend.ai-dev2/dist/export/python/virtualenvs/python-default/3.11.6/lib/python3.11/site-packages/sqlalchemy/engine/base.py", line 655, in _revalidate_connection
    raise exc.ResourceClosedError("This Connection is closed")
    ^^^^^^^^^^^^^^^^^
sqlalchemy.exc.ResourceClosedError: This Connection is closed

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Documentation
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to demonstrate the difference of before/after