Add Mongo trait.

MongoController can be mixed only in Controller. Add Mongo trait that can be
composed into any Scala class.
Rename SystemMetricsActor to SystemMetricsMonitor.

SystemMetricsMonitor is a more appropriate name because it is used to monitor
different types of system metrics.
Introduce SystemMetricsSupervisor.

This patch makes SystemMetricsMonitor a child of SystemMetricsSupervisor.
SystemMetricsSupervisor will supervise system metrics related actors.
Write system metrics to the database through SystemMetricsWriter.

Make SystemMetricsSupervisor query the current system metrics every 10 seconds
and write them to MongoDB's "admin.metrics" collection through
SystemMetricsWriter.