Smarter treatment of recipients that are collections

- Allow client to specify recipients that don't need to be delivered to
- When fetching recipient, recognize collections and don't try to deliver to
  them
- Remember collections in DB, and use that to skip HTTP delivery

migrations/2019_05_17.model

@@ -0,0 +1,5 @@
+RemoteCollection
+    instance   InstanceId
+    ident      Text
+
+    UniqueRemoteCollection instance ident