Track pending MUX logins via memcache

Track pending MUX logins via memcache

When a SIP child process is spawned to handle a new connection login,
the pending login is tracked in the parent process (by PID) and the
child indicates to the parent that the login has succeeded by storing
login success/failure plus some state information in memcache.  Any time
the parent wakes up to process a message, it checks for completed logins
so they can be resolved as OK in the parent and the state information
is extracted and stored for future conversation with the resolved client.

Signed-off-by: Bill Erickson <berick@esilibrary.com>