MINOR: server/event_hdl: add support for SERVER_UP and SERVER_DOWN events

MINOR: server/event_hdl: add support for SERVER_UP and SERVER_DOWN events

We're using srv_update_status() as the only event source or UP/DOWN server
events in an attempt to simplify the support for these 2 events.

It seems srv_update_status() is the common path for server state changes anyway

Tested with server state updated from various sources:
  - the cli
  - server-state file (maybe we could disable this or at least don't publish
  in global event queue in the future if it ends in slower startup for setups
  relying on huge server state files)
  - dns records (ie: srv template)
  (again, could be fined tuned to only publish in server specific subscriber
  list and no longer in global subscription list if mass dns update tend to
  slow down srv_update_status())
  - normal checks and observe checks (HCHK_STATUS_HANA)
  (same as above, if checks related state update storms are expected)
  - lua scripts
  - html stats page (admin mode)