Move stream-event JSON objects to a common package

This way they aren't so tied up with the ChangeHookRunner itself,
and we can start to reuse them for a gerrit query implementation.

Change-Id: If531bc193e3569f7df626c303cc8dc54be237ee0
Signed-off-by: Shawn O. Pearce <sop@google.com>
14 files changed