Switch to ReadPreference.secondaryPreferred() for the river connections

We're overloading the primary at the moment, to the point where some queries never get processed
at all (this is what gives the timeout exceptions, and what leads to the high number of CLOSE_WAIT connections
on the primary).

Probably the fix is manifold:
1. MongoDB should still send keep alive messages for the cursor, even when the query itself
   is waiting for a lock
2. The river should only do a single query on the oplog, and distribute the relevant information
   internally to the Indexer threads
3. We generally need to reduce the load on the server :)
Merge branch 'elasticsearch-river-mongodb-1.6.9-collaborne' of https://github.com/Collaborne/elasticsearch-river-mongodb into elasticsearch-river-mongodb-1.6.9-collaborne