HPCC-11522 libmemcached plugin

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
It does. So it made sense to me for MemCachedPlugin::MCached::getKeyType to return an actual eclDateType enumeration rather than an int that would need to be cast later down the line (though this would be a trivial fix to the issue I'm about to explain). A fix would also be to add an extra to this enum list to handle this case, seemed like a hack at first but perhaps not. The easiest is to simply remove the extra exist and then have the aasertError within  MemCachedPlugin::MCached::getKeyType return a 'NOT FOUND' error message. The first question is whether we want the assertion, i.e. I would have thought a better behaviour would be to not terminate. But then.... it doesn't make sense for the internal to fail either, I think I'll add a 'NONE' to eclDateType.