* Fix race condition on read/write of shutdown bool variable of server and connection pool.
* In connection pool, there is no guarantee that .reap() cannot execute the same time as .Shutdown() is called. It also did not benefit to eval shutdown when a select is run on the shutdown channel.
* In server, same principle applies to handleConsulConn. Since we also have a shutdown channel, it makes more to use this than to loop on a bool variable.
Merge pull request #499 from alouche/fix_potential_race_condition_shutdown

Fix potential race condition on shutdown (pool.reap/server.handleConsulConn)