DB/Index Redux

This change removes the timestamp field from messages and conversations
in favor of multiple semantically named timestamp fields: sent_at,
received_at on messages; active_at on conversations. This requires/lets
us rethink and improve our indexing scheme thusly:

The inbox index on conversations will order entries by the
conversation.active_at property, which should only appear on
conversations destined for the inbox.

The receipt index will use the message.sent_at property, for effecient
lookup of outgoing messages by timestamp, for use in processing delivery
receipts.

The group index on conversation.members is multi-entry, meaning that
looking up any phone number in this index will efficiently yield all
groups the number belongs to.

The conversation index lets us scan messages in a single conversation,
in the order they were received (or the reverse order). It is a compound
index on [conversationId, received_at].

js/views/message_view.js

@@ -73,7 +73,7 @@
         this.$el.html(
           Mustache.render(this.template, {
             message: this.model.get('body'),
-            timestamp: moment(this.model.get('timestamp')).fromNow(),
+            timestamp: moment(this.model.get('received_at')).fromNow(),
             bubble_class: this.model.get('type') === 'outgoing' ? 'sent' : 'incoming',
             sender: this.model.get('sender')
           })