HPCC-11522 libmemcached plugin

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Well it's within the namespace MemCachedPlugin and the other refs are not so I'd have to use - 
MemCachedPlugin::OwnedMCached which kinda defeats the point of being lazy. If we don't like this I'll just take it out altogether, I could add another one external to the namesapce that includes this, but this seems even messier.