HPCC-11522 libmemcached plugin

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Additionally, since data can be set via something other than this plugin and thus set with what ever it wants for the flag, is there a way to discern this, i.e. that it was set from this plugin in the first place? Ultimately I can't see how you could (since the flag is jsut an unsigned int) but perhaps there is a way to make it it more obvious, e.g. hardwire the enumerations to a 'non-standard' part of the number sequence?
@richardkchapman, any suggestions?