Synchronisation introduced by HIVE-14296 on SessionManager.closeSession causes high latency in a busy hive server

HIVE-14296 introduces this commit which adds synchronization to SessionManager.closeSession.

And it looks like it is used only for logging purposes.

In a busy hive server where 5-10 sessions are created closed every second, an increase in latency of any other downstream services (Zk, HDFS) causes a queueing effect (lot of threads getting blocked on SessionManager.closeSession) creating an induced latency of 3-5 minutes at times for just closing the session. 

Since the gauge (MetricsConstant.HS2_OPEN_SESSIONS) is already tracking the open session counts, the synchronization (along with the additional logging) can be removed without any functionality losses.