This enables deciding at startup time if hooks will be supported,
and if not stub in a dummy implementation that does nothing. This
is mainly useful for my gerrit-review.googlesource.com environment
where the standard hook system of spawning an external program is
not supportable.
The factoring is a simple step towards a more pluggable event system.
Someone could implement a new version of ChangeHooks that uses a
Guice multibinding to dispatch these events to multiple instances
of the ChangeHooks API.
Change-Id: I6e9fba78f87d6e9ba8aaf80d386230334a492ea2
8dba895ad6