mon: HealthMonitor: Keep track of monitor cluster's health

mon: HealthMonitor: Keep track of monitor cluster's health

The HealthMonitor builds upon the QuorumService interface, and should be
used to keep track of all and any relevant information about the monitor
cluster (maybe even about all the cluster if need be).

This patch also introduces the HealthService interface, used to define
a HealthMonitor service, responsible for dispatching 'MMonHealth' messages
(the QuorumService interface dispatches generic 'Message').

Based on the HealthService interface, we introduce the DataHealthService
class, a service that will track disk space consumption by the monitors,
warn when a given threshold is crossed, and gracefully shutdown the monitor
if disk space usage hits critical levels that might affect the correct
monitor behavior.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>