HPCC-11522 libmemcached plugin

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
I wonder if these should remove the M prefix.  The scope is already qualified by memcached.

The reason for bringing it up is that if we also provide a redis implementation it would make sense for the functions to be called the same - assuming they can have the same prototypes.