HPCC-11522 libmemcached plugin

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
The plugins for database access took the approach of NOT having separate calls for all the different sizes, though did end up needing to backtrack on that for a few cases where the size of the field was set externally, and there was no way to determine what the size was. If that could apply here, then we will need SOME way of indicating the size.