use monotonic consistency

- using monotonic consistency allows us to connect to secondaries.
  Since we only do reads, this has no downsides over the 'strong'
  consistency level
my understanding is that if the connection string has `?connect=direct`, then mgo will not discover any other machines in the replica set, and you must read from the machine specified in the rest of the connection string.

the benefit of this line seems to be if you *don't* specify `connect=direct`, then mgo will discover all machines in the replica set but only choose to read from secondaries.