Add logging information about the pruning result.

[jameinel]

This gives a bit nicer set of information at INFO level. That doesn't
overwhelm you with the details, but can give you a general idea of what
has been done.

As part of doing manual testing with accelerated pruning, I felt it would be nice to really see what was going on. (running cs:~jameinel/peer-xplod with 24 units spread across 4 containers is a decent load test. I'd like to understand why jujud-machine-0 is at 236%CPU while mongo is only 33% CPU, but that's for another day.)

The log messages look like:

machine-0: 17:37:19 INFO juju.txn txns after last prune: 5, txns now: 122, pruning required: true transactions have grown significantly
machine-0: 17:37:20 INFO juju.txn txn pruning complete after 412.843118ms. txns now: 4, inspected 56 collections, 51 docs (46 cleaned)
   removed 0 stash docs and 120 txn docs
machine-0: 17:37:40 INFO juju.txn txns after last prune: 4, txns now: 122, pruning required: true transactions have grown significantly
machine-0: 17:37:40 INFO juju.txn txn pruning complete after 294.31639ms. txns now: 2, inspected 56 collections, 48 docs (47 cleaned)
   removed 0 stash docs and 121 txn docs
machine-0: 17:38:00 INFO juju.txn txns after last prune: 2, txns now: 114, pruning required: true transactions have grown significantly
machine-0: 17:38:00 INFO juju.txn txn pruning complete after 416.572073ms. txns now: 3, inspected 56 collections, 42 docs (38 cleaned)
   removed 0 stash docs and 112 txn docs

(Review request: http://reviews.vapour.ws/r/6517/)