Add authentication counters in ns_server

In order to make it easier to understand how many authentications per second a node is running, we need to add a separate counter for it.

We currently can get most of this data indirectly (by the number of http requests for example), but that data is incomplete. For example, it will miss authentications coming from memcached (external auth), and authentications coming from cbauth.

It also would be useful to be able to get breakdowns by type of authentications (local, external, token, ...), and by result (succ or failure).