Move setMeta in to memcahed

Currently getMeta is implemented under memcached due to MB-25427, which makes sense in terms of the code base. As part of this implementation, the command was interfacing with the normal get stats, which also makes sense as getMeta is a flavour of get and so we should be able to see the usage of it. However, due to MB-28206 we had to remove this behaviour as it was causing the getMeta commands to be counted twice in terms of the stats shown by the UI. By refactoring setMeta into memcahed, we can have this behaviour for both commands, and then stop ns_server from counting them twice once we have the consistency.