MB-9307: Take unindexable partitions into account

Partitions might be set to unindexable, but that doesn't mean
that we can't roll back to them and use them as a starting point
for future updates.

Change-Id: I401e4ea0de6f5c8f95c823cd67f780b159860413
Reviewed-on: http://review.couchbase.org/30020
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Volker Mische <volker.mische@gmail.com>