Fix in-memory queue using shared-cache

When share in-memory queue in multi-thread, the queue is not
properly synced between threads, this commit uses shared-cache
to achive the synchronization.
fixes #31

persistqueue/sqlbase.py

@@ -97,9 +97,8 @@ def _init(self):
         self._conn.commit()
         # Setup another session only for disk-based queue.
         if self.multithreading:
-            if not self.memory_sql:
-                self._putter = self._new_db_connection(
-                    self.path, self.multithreading, self.timeout)
+            self._putter = self._new_db_connection(
+                self.path, self.multithreading, self.timeout)
         if self.auto_commit is False:
             log.warning('auto_commit=False is still experimental,'
                         'only use it with care.')
@@ -110,10 +109,10 @@ def _init(self):
         self.put_event = threading.Event()
 
     def _new_db_connection(self, path, multithreading, timeout):
-        conn = None
         if path == self._MEMORY:
-            conn = sqlite3.connect(path,
-                                   check_same_thread=not multithreading)
+            conn = sqlite3.connect(database='file::memory:?cache=shared&model=memory',
+                                   check_same_thread=not multithreading,
+                                   uri=True)
         else:
             conn = sqlite3.connect('{}/data.db'.format(path),
                                    timeout=timeout,