Add connection validity check in acquire method

[lxneng]

• A new asynchronous method _check_conn is added to check the validity of the connection. This method is used in the acquire method to ensure only valid connections are acquired from the connection pool.
• Change the condition in initialize method to check freesize instead of size.

[itssimon]

This is a good idea. I'd make it opt-in though.

SQLAlchemy uses the pool_pre_ping argument (default False) to enable this behaviour.

https://docs.sqlalchemy.org/en/20/core/pooling.html#disconnect-handling-pessimistic

[long2ice]

Thanks! Please resolve conflicts first.

[lxneng]

Thanks! Please resolve conflicts first.

@long2ice I have resolved all the conflicts. Please review this PR again. Thank you!

[long2ice]

Thanks!