HPCC-11522 libmemcached plugin

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
Yes I know, I had no idea where to add this and I did ask. Adding it to the plugin makes no sense either due to maintenance of code numbering. I would have thought there to exist a similar err/wrn code list for the runtime lib but couldn't find anything.