Fork a hook process, instead of forking every time.

[sargon]

We could fork a process early and then pass events by a unnamed socket.
This would solve the issue with multiple hook script instances, forking
on every event and prevent and resources over consumption by an event
spike.

[christf]

another way would be starting one process like run-parts that runs all the hooks.

EDIT: actually the run-parts approach does not solve the issue entirely. hooks for different events are still processed in parallel.