Perfmon for number of open client connections

[danielmewes]

Would be very useful for investigating memory usage issues, and also for general monitoring.
I don't think we have that yet, do we?

Suggested by @victorquinn in #2988 (comment)

[Tryneus]

As I mentioned in #2988:

Currently [...] the closest thing we have is in the perfmons in /ajax/stat [...], each server has a query_language field. I think we should add a couple more fields like open_connections and cached_streams, which would ease debugging in situations like this.

[danielmewes]

@Tryneus has implemented this and it's available through r.db('rethinkdb').table('stats') in the query_engine.client_connections field.