* Add request pool to improve delivery performance
Fix#7909
* Ensure connection is closed when exception interrupts execution
* Remove Timeout#timeout from socket connection
* Fix infinite retrial loop on HTTP::ConnectionError
* Close sockets on failure, reduce idle time to 90 seconds
* Add MAX_REQUEST_POOL_SIZE option to limit concurrent connections to the same server
* Use a shared pool size, 512 by default, to stay below open file limit
* Add some tests
* Add more tests
* Reduce MAX_IDLE_TIME from 90 to 30 seconds, reap every 30 seconds
* Use a shared pool that returns preferred connection but re-purposes other ones when needed
* Fix wrong connection being returned on subsequent calls within the same thread
* Reduce mutex calls on flushes from 2 to 1 and add test for reaping
