HPCC-11522 libmemcached plugin

Previously: #6368

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
@ghalliday Here lies the oddity that is. We don't really want to use this so that the user may do so by passing the relevant info in the config string along with the server info. That being said, there is a bug in libmemcached regarding these 'namespace' and 'hash_with_prefix' options when passing them in as a string, so that can't use it unless we doc it or they take a look at the source. If I remember, this one is not a bug but more of a documentation/source discrepancy.

The reason I added this is. I thought it solved the server group partition issue I was having however, it does not. It did, but this was a façade, it depends on the length of the string used here for the prefix, whether it finds the key or not!