Replace synchronized block by a ConcurrentMap to reduce the contention on connection acquisition

Currently the code to get a connection from a datasource passes through NuxeoConnectionManager$ActiveMonitor.handleObtained which is a synchronized method. The synchronized block is here to protect the concurrent update of the ttls map.

It seems that the synchronized block could be removed and replaced by the use of a ConcurrentMap for the ttls variable to reduce the possible contention when acquiring a connection.