Decrement egress counter when scoped connection not returned to the pool

this code calls kill() in certain cases and done() in others. Only done() decrements the _checkedOut value.

The result is that openConnections() rises inaccurately over time.

The inaccurate reporting was observed on 3.6.7 but code appears unchanged since. It's understood that PM-1121 may eventually obviate this pool but it may not be a difficult fix in the interim.