OPNFLWPLUG-1090: ConcurrentModificationException: null when connecting cbench switches

OPNFLWPLUG-1090: ConcurrentModificationException: null when connecting cbench switches

Issue:

CSIT job using cbench was frequently failing because some switches are
getting their initial connection rejected.

Observation and Fix:

On analyzing the logs it was found that the failure had been caused by
the sessionstats collection during initial handshake. "ConcurrentModification"
exception was thrown as multiple switch sessions were trying to modify
the SESSION_EVENTS map which reulted in the connection getting closed.

This has been fixed by making the SESSION_EVENTS threadsafe now.

Change-Id: I2d0ec7c00d7c893d6271942f5fdbe9815069d4d0
Signed-off-by: Gobinath <gobinath@ericsson.com>
(cherry picked from commit bd28cb92e0f6f3bca79a04a5e5a57c617b6cb552)