HPCC-11522 libmemcached plugin

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
I wasn't sure about this. At present this only checks when creating the connection and not when it uses a cached connection. If all servers drop afterwards then this will be reflected in a failed connection via the plugin function called. I expect the best thing might be to roll into one with a severity mapping that can be changed upon warning about those that have dropped. The bit I wasn't sure about is, if my latter suggestion is a good one then is this too much runtime effort if we should wan to check again when getting the cached connection?