Be defensive when calcualting pending_count
Be defensive when calcualting pending_count

Its theoretically possible that during the upgrade we end up with a
mixed set of RPC workers. This makes sure that we don't crash the
coordinator with a badarith error.

BugzId: 24236

fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb,fkaempfer/couchdb