It is better to have type-specific event names to minimize the amount
of constraint string matches we do on hooks, as most hooks (if not all)
are interested on specific types of objects only.
Similarly, use a different object manager for each object type to
minimize the performance impact of iterations and lookups, as all
such actions are interested in only 1 object type every time.
Port all existing hooks to the new event names and the get-object-manager API.
This avoids having to determine the subject type prior to pushing
an event from lua code and makes the call more convenient
Also add a debug statement to trace calls to push_event
This can be used anywhere else in the codebase to push a "rescan-session"
event, making sure that there is only one such event pushed on the stack,
no matter how many times this is called.
