Consider making the `close` method of `Connection` and the `flush` and `close` methods of Appender async

Closing a connection or flushing or closing an appender could result in nontrivial work. Perhaps these methods should be made async.

One complication is when a connection or appender object is finalized and gets closed in the finalizer (which can't be async). Perhaps both sync and async methods could be provided for this functionality.