Custom mailbox that is bounded and instrumented.

This is for use with Akka actors. It enables monitoring
of actor's mailbox size via JMX.

Change-Id: Ic1e478e4411f53ff0239d316a1ca02eaa80360ed
Signed-off-by: Abhishek Kumar <abhishk2@cisco.com>
Merge "Custom mailbox that is bounded and instrumented."