rework request invalidation

rework request invalidation

This kills 2 birds with 1 stone: Removes the necessity of locks
and makes call_slot() realtime safe (req->invalidation->requests list
push_back). On object destruction, the invalidation-record (IR) itself is
invalidated.

Invalidated IRs are pushed onto a trash-pool and deleted in the event-loop
of the invalidated object (GUI thread) once all requests that reference it
have been processed.

One last detail remains: PBD::signal connect should reference the IR
and disconnect unreference it. This will guarantee that signal emission
will not reference the IR while the pool trash is dropped.