Split stats into standalone service

[georgesnelling]

Currently collections have no protection from concurrent rebuild / refresh commands.

We refresh stats every minute on the service using a recurring task. When the stats are screwed up we issue a /stats/rebuild command manually. If this doesn't finish within a minute the recurring refresh command merrily starts up and chaos ensues. The most common symptom is double values.

Stats should be split out as an entirely separate service with its own database that includes the current tos and froms persisted map-reduce collections. Rebuilds should create temp collections and hot-swap them when complete. Both refreshes and rebuilds should be protected from concurrent starts, and should provide kill commands.