Balance replication ownership across nodes

The previous algorithm was biased towards low-numbered nodes, and in the
case of a 3 node cluster would declare db1 to be the owner of all
replications.  We can do better just by leveraging the existing
ushards code.

There's a possibility to refactor this as a new ushards/2 function if
that's perceived as useful.

BugzID: 19870
Merge pull request #51 from cloudant/19870-improve-replication-balancing

Balance replication ownership across nodes

BugzID: 19870