Emit warning if connection is deleted before it is closed

[beer-psi]

Description

An unclosed aiosqlite.Connection causes the interpreter to stay alive even after the main thread exits.

This PR fixes it by

• Making the Connection owns a thread (instead of be a thread)
• The worker thread function is moved out of the Connection class and only receives the shared request queue
• Implement __del__, which emits a ResourceWarning if the connection is still active, and attempts to stop the worker thread.

Changes 1 and 2 makes it so that the thread does not hold a reference to the entire connection object, which allows the object to be garbage collected when unused (unless some other thing is causing a reference cycle e.g. exceptions) and __del__ called.

Unlike #308, the worker thread is not daemonic, so there shouldn't be a risk of the thread being shut down before transactions are finalized.

This is likely a breaking change, since Connection is not a subclass of Thread anymore.

Fixes: #290 #299