Pinger should batch ping requests to the database

[John A Meinel]

Pinger currently uses a separate mgo.Session for every ping (possibly sharing the connection underneath).

However, all the pings for a given bucket could *easily* be handled in a single Increment, rather than creating an $inc for each request. (this is because all of the requests for the same bucket can add together in memory instead of in the database.)

This would reduce our total number of operations on the database as well as the maximum number of concurrent connections and total round trips.

[John A Meinel (jameinel)]

https://github.com/juju/juju/pull/7531