Plugins can inject external events into Gerrit
also coming from other remote instances. Preserve
the instanceId contained in the forwarded event
if set and do not overwrite with the local Gerrit one.
Only events that are locally generated in Gerrit
can set the intanceId but not the forwarded ones.
Failing to preserve the original instanceId of the
forwarded events may cause events duplication, like
the use-case of the high-availability plugin not
recognising a remote event because it is re-stamped
with the local instanceId.
Bug: Issue 13307
Change-Id: I324a48a6f2ffca59bad059e18550a8dc44224f34
c862d29ece