Don't attempt to replace replacement shards

The old test was looking for {_, _} tuple but that doesn't account for
old since sequences. This just switches to using a negative selection.
The only bad behavior was that we were making lots of log messages about
rewinding from 0 in known conditions.

BugzId: 26125
Merge pull request #137 from cloudant/26125-fix-changes-replacement-log-bug

Don't attempt to replace replacement shards