Make check module use callback events for hosts and service checks

The checks.c file used quite nasty interface for the event module,
tweaking raw parameters in the timed_event struct, which should be an
opaque type for the events.c module.

Let checks.c take over the functionality of lot of host/service check
specific filters in the events.c module, use callback events, but still
some event-time access is done. This does however mean that the
events.c can start getting refactored to remove lot of event-type
specific logic.

Signed-off-by: Max Sikstrom <msikstrom@op5.com>