Change write quorum error logs into metrics

The global_changes work uncovered the fact that the common "write quourm
failed" log messages happen for run of the mill conflict errors. Rather
than try and change the logic around to fix the error this just adds
three metrics to cover the events so that we can monitor it in the
future.

BugzId: 23977
Merge pull request #120 from cloudant/23977-fix-quorum-error-log-message

Fix the write quorum failed log message