Statistics module

A simple statistics module that allows other modules to push statistics
on a by tenant basis. Eventually there’ll be a possibility to push stats
into statsd.

Currently only a content storage stat has been implemented. This stat
displays the total storage used by a tenant in bytes. The storage stat
collector can be enabled and configured in config.js.
Current configuration options:
 * enabled: Whether or not the collector should run on this node
 * interval: The interval when the stats should be updated
 * expiry: How long a collection can take before something is considered awry

By default only one node in the cluster can refresh the content storage
stats.