Multiplex should fail definitively if memcached not running

[Galen Charlton]

We recently had a situation where we were running SIPServer in Multiplex mode without a connection to a running memcached server.

While that situation doesn't outright stop SIPServer from running, it does degrade responsiveness and leads to connection errors as perceived by the SIP2 clients. Consequently, it might be best if SIPServer at minimum verifies on startup that it has a good connection to memcached, and refuse to run if not.