🚀 performance: add concurrency to all search/find functionality, lazy imports, etc

[epwalsh]

• Adds an async module with an abstract Executor class that mimics Python's concurrent.futures.Executor.
• Adds two implementations of the Executor class: AsyncExecutor and ThreadPoolExecutor, which use coroutines and the Libuv threadpool, respectively, to run user functions concurrently.
• Uses the AsyncExecutor in cmp_obsidian and other places that rely on search/find functionality.