HPCC-11522 libmemcached plugin

Previously: #6368 & #6686

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
returnValue isn't initialised at this point.

trivial: msg.append((unsigned)sizeof(type)) would possible be more normal.  As it stands you may get a warning about losing accuracy (sizeof(long) == 4).