MDL-48506 cachestore_memcached: added shared cache config option

MDL-48506 cachestore_memcached: added shared cache config option

When added a memcached instance you can now select whether the cache is
being shared by other applications. The setting will determine the
purging strategy.

Shared caches will have individual keys deleted while dedicated caches
will have the entire cache purged (better performance over networks).

Note: This option only works with the correct version of the php
memcached extension and with the multi-site safe changes.