Add (generic) support for providing arguments and options to a stats call

The current definition of the stats call is that it only allows for a KEY in the value, and that is the stats we're requesting. We've worked with that by adding a new stats group if we want slightly different output, and some commands use a space to take a single argument.

We've added a lot of the stats groups to be used for kv developent (or to gather information when we've run into trouble). Ns_server use some of these groups to monitor the system, and end up discarding a lot of the information we build up and send back. By allowing for arguments to be passed along with the stats call we may add filters to the various stats groups (for instance the dcp stat could return just DCP connections bound to a certain port or user).

The current proposal is to update the binary protocol specification to state that a STAT call MAY contain a value, and if its present it needs to be JSON. We don't want to enforce a schema, but leave that up to each stat (we should however TRY to standardize where it makes sense).