Add a new extension point so that plugins can be notified when a change
has been indexed or removed from the secondary index.
This allows the new Sync-index plugin to keep the secondary index of two
Gerrit instances in sync. Listening to existing change/patch events
cause the plugin to miss other cases where index updating is done, e.g.,
deleted drafts or changes reindexed to update their mergeability status.
Change-Id: I87de17de26e907d77b2fb5175b5c90c5dd0381e9
5fbbdadd65