Ignore {ddoc_updated, _} couch_db_update events

There's a couch_db_update event handler defined in
fabric_db_update_listener. This event handler is only used for clustered
_changes feeds. I think it should be fine to only match against
{updated, DbName} events and drop all other events (including
ddoc_updated).

BugzID: 10979
Merge pull request #84 from cloudant/10979-update-all-shards

Index all shards on ddoc update