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/inbox_view.js

@@ -32,11 +32,15 @@
                 collection : this.conversations
             });
 
-            this.conversations.fetch({ reset: true }).then(function() {
-                this.conversations.reset(
-                    //TODO: Add an index to support this operation at the db level
-                    this.conversations.filter(function(c) { return c.get('active'); })
-                );
+            this.conversations.fetch({
+                index: {
+                    name: 'inbox', // 'inbox' index on active_at
+                    order: 'desc'  // ORDER timestamp DESC
+                },
+                reset: true
+                // TODO pagination/infinite scroll
+                // limit: 10, offset: page*10,
+            }).then(function() {
                 if (this.conversations.length) {
                     this.conversations.at(0).trigger('render');
                 }