BUG-7768 Synchronize ServerSessionManager for PCEP
Add the check so that session manager can only be closed once
and becomes unusable after closing.
A java.util.ConcurrentModificationException is possible to happen
due to the close() method of ServerSessionManager is not synchronized.
The situation mostly likely will happen during a controller reboot.
It may also happen when a PCEP config is updated. The ServerSessionManager
is being restarted while all the PCEP negotiation is still happening.
Change-Id: I0e0d387add046a4fe9ec5ac74a85f8350e238d60
Signed-off-by: Kevin Wang <kevixw@gmail.com>