Stop initializing new SSL connection if ZK server is shutting down

ZK keeps accepting new connections while it's being shut down then immediately close them when it finds out that the ZK server is not running. It's not a big deal before SSL is enabled since creating TCP connections is relatively cheap.
 
With SSL being widely enabled,  creating SSL connections involves handshake that takes non-trivial CPU time, which is wasted since the connections are closed right after. 
 
This JIRA is going to stop initializing TLS handshake if the zkServer is not serving to save resources.