Emit 'release' event and add 'activeCount' gettable property to pool

[PrimeJunta]

Currently the pool only tracks total clients and pending connections. It would be extremely useful to be able to know how many clients are checked-out (active) at any given time, as this would make it easy to create unit tests specifically to test for bugs that leak clients.

[ghost]

You can just do pool.totalCount - pool.idleCount to get the active client count