Replace random balancer with roundrobin.

Some changes have been made:

* Naive random load balancer was hard to test and had poor performance, so it was replaced with round robin everywhere
* Balancers have been moved to a separate package
Make better comments
Change default replication factor
Fix upstream ids
One more fix
Compatibility fix
Merge pull request #14 from mailgun/sasha/roundrobin

Sasha/roundrobin