Add query_db_wait_timeout configuration option

[james727]

Hello PgBouncer team. I'd like feedback on a new configuration field. This adds a parameter (query_db_wait_timeout) to address the problem described in in #585.

Setting query_db_wait_timeout to a nonzero value will disconnect a client if the following two things are true:

1. It's been waiting to be assigned a server connection for longer than query_db_wait_timeout
2. The underlying pool is detected to be in a fast-failure state, as determined by existing logic for fast failing new connections.

Note that this does not require the pool to have been in a fast failure state for query_db_wait_timeout seconds, just that the client has been waiting and the pool is currently in a broken state when the bookkeeping occurs.

Thanks in advance.