Use stream events to fetch refs and create repositories

Add stream events listener to trigger git fetch references and to create
repositories. This functionality in combination with events-broker and
multi-site provides a backfill mechanism for REST API calls missed when
node was unreachable.

NOTE: This change do not include removing refs, project deletion and
head update.

Bug: Issue 15636
Change-Id: I573d4b403878ad781eeabf2b88416f3b29c25787