HPCC-11522 libmemcached plugin

Previously: #6368 & #6686

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
As far as I can see this is actually ignored if pb->ECL is NULL.  However I don't see why it isn't lib_memcached like all the other plugins - including those that have a separate ecllib.