mon: QuorumService: Allow for services quorum-bound to be easily created

As the monitor grows in features, we have been dumping them in the Monitor
class as they don't really fit anywhere else.

Most of those latest features have been, and some of the future changes
will also be, quorum-bounded.  By that we mean that these features tend
to require a quorum to be present in order to work.

Although we already have the PaxosService interface, it really isn't
adequate for this kind of features, as they don't really require Paxos,
nor do they access the store.  Furthermore, they don't really need to
tick at the same rate as the monitor, and can be fairly independent.

Therefore we now introduce the concept of a QuorumService, a class to be
built upon, managing the tick and dispatch for any kind of service
basically requiring a quorum to function.

Among the already existing monitor features that could take advantage of
this new class we can find the Timecheck infrastructure, as it is by
nature quorum bounded.  The monitor store sync could also take advantage
of this service, although it doesn't really require a quorum to work,
and even the PaxosService-related classes could use this.

This patch also introduces the MMonQuorumService base class, to be used
by any message that should want to.

Signed-off-by: Joao Eduardo Luis <joao.luis@inktank.com>
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>
mon: Monitor: take advantage of the new HealthMonitor class.

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