Add AcquireTimeout to Pool

[typokign]

Adds a timeout for acquiring a free connection from the connection pool.

This existed in previous versions of PGX: https://github.com/jackc/pgx/blob/v2/conn_pool.go#L29.

This is useful for cases where queries may be slow, but acquiring a connection from the pool is expected to be fast, and it is desirable for requests to fail fast if the connection pool is starved. This is possible today by manually calling Acquire() for every query and passing a context with a shorter timeout, but there is no way to configure a separate timeout for Acquire() when using the Query(), QueryRow(), Exec(), etc. helper functions on Pool.

[jackc]

I'm not opposed to this feature overall, but I have a few concerns about the implementation.

1. It appears that when the acquire timeout is triggered, the error returned will be a context.DeadlineExceeded. That means a caller can get a context.DeadlineExceeded error when the context they passed in is still valid. The ambiguity is unfortunate.
2. There's no way to override on a per call basis. Should be a rare occurrence, but the lack of symmetry is again unfortunate.
3. I could imagine that someone may want something similar like a default query timeout for the query execution itself. Something like a pgx side statement_timeout. Whatever approach is used it would be nice to have a pattern that works for all similar features.