Add /mark_all_as_read endpoint.

This change simplifies how we mark all messages as read.  It also
speeds up the backend by taking advantage of our partial index
for unread messages.  We also use a new statsd indicator.

static/js/unread_ops.js

@@ -10,12 +10,8 @@ exports.mark_all_as_read = function mark_all_as_read(cont) {
     unread_ui.update_unread_counts();
 
     channel.post({
-        url:      '/json/messages/flags',
+        url:      '/json/mark_all_as_read',
         idempotent: true,
-        data:     {messages: JSON.stringify([]),
-                   all:      true,
-                   op:       'add',
-                   flag:     'read'},
         success:  cont});
 };