Added metric to monitor the connection-pool size.

Moved companion object
Counting the sum of connections to each endpoint instead of only each
endpoints.

Alexified !
Added more metrics;
* The size of the map (pooled destination)
* Pool hit/miss
* Addition and removal from pool
No more return and no more var !

Alex solution is always better...
Using AtomicInteger the right way to count the number of pooled connections. (and returns it as a metric instead of iterating throught the connections map)
Merge pull request #159 from wajam/jerome-connectionpool-metrics

Added metric to monitor the connection-pool size.