HPCC-11522 libmemcached plugin

Signed-off-by: jamienoss <james.noss@lexisnexis.com>
So every time a plugin function is called a connection is opened and closed, created and deleted. I wouldn't have expected this to slow anything down. The bulk of the code in the constructor would have to be called regardless of the connection being kept alive or not. The reason for this is that the server list is passed from the plugin functions on every call and therefore all checks need to be carried out every time anyhow. The reason that the server list has to passed every time is so that you can obviously control which cache you wish to connect to but also such that you can also dynamically add more (startup another daemon listening on  another port or just an extra server) at any time. I could make it static (in fact I did at the beginning) and then run through the memcached_st for the current server lists, comparing this to the one just passed to it from the plugin function. I don't think this would be any better and to have multiple memcached caches I'd then need a list of static connections.