Deadlock when opening a 2nd connection with an unclosed statement from previous connection

[juja0]

Not sure if this is expected behaviour but if a new connection is opened when a prior (closed) connection has an unclosed statement, the request hangs.

Properly closing all statements before closing connections seems to fix the deadlock.

Could potentially be related to the busy-wait added in this PR.

Wanted to check if it would be better to have a configurable timeout for the busy wait since the cause of deadlock may be hard to pin down when it hangs.

Sample repro: https://github.com/juja0/duckdb_deadlock