Fixed pecl memcached client in persistent mode.

Fixed pecl memcached client in persistent mode.

* When using a persistend ID in the constructor, the object is effectively shared among
  the callers using that ID. If we always use __CLASS__, then this breaks when different
  BagOStuffs may be used. Instead, a hash of the settings is now used.
  See http://www.php.net/manual/en/memcached.construct.php.
* Don't keep adding servers to the pool, as the object is shared among callers. Once one thread
  initializes the object, it will already be initialized by the next thread. Calling addSever()
  again will cause an increasing number of duplicate servers to exist in the data structure, and
  thus an increasing number of socket connections over time.
* Also, use addServers() instead of multiple addServer() calls per
  http://www.php.net/manual/en/memcached.addserver.php.

Change-Id: I0e7510320cb79d9f152e8958ddd50400fa9da37f