Handle duplicate docs in bulk update

Previous to this commit, fabric assumed that all docs in a bulk update
request were unique. In the case that they were not unique, fabric would
error and return a stack trace to the user. This commit makes fabric
identify docs in a bulk update by a ref rather than their id.

This commit is based on work done by Bob Dionne.

BugzID: 12540
Merge pull request #116 from cloudant/12540-dup-updates-in-commit-davisp

Handle duplicate docs in bulk update