Icinga now sends runtime updates in two separate channels,
icinga:runtime for config updates and icinga:runtime:state for
state updates. With this change, Icinga DB reads from these two
streams. This is a preparation so that state updates can be
streamed directly after a (re)start of Icinga or Icinga DB without
waiting for the config sync, as it is currently done.
Heartbeat now uses pointer receivers for its methods because
some methods actually change the heartbeat values.
The context is no longer stored in the structure,
but passed to the controller loop.
The beat and the lost channels are replaced by Cond and
the last heartbeat is stored independently to not be affected by
a slow HA receiver. If the database connections are occupied by
the config, HA cannot update the instance and does not read from
the beat channel in time.
In addition, heartbeat errors are no longer swallowed,
but handled in HA.
