#1346 Fix race condition in bolt connection pool

The pool tries to acquire an idle connection first. If no idle connection is
available, it puts itself into a queue waiting for another thread/task to return
a connection before trying again.

However, there is no protection against a connection being returned between the
attempt to acquire an idle connection and the enqueuing. Therefore, it could
happen that a thread waits indefinitely like this:

Assume max pool size of 1

• T1 creates & acquires con1
• T2 checks for idle connections => None
• T1 releases con1
• T2 acquires lock to be able to wait for the release event
• T1 terminates
• T2 times out waiting for Godot

Closes: DRIVERS-578