Create a messaging channel among PeerDB instances.

Use it for distributing updateAll calls. Breaking change: we are
renaming internal "migrations" collection to "peerdb.migrations". We
are introducing additional one: "peerdb.messages".
Use TTL index instead of capped collections

Capped collections made the whole Meteor app very very slow. So let's
use TTL index which auto expires messages instead. And then normal
Meteor observe.
Version bump.