Add phase item to view compaction status
Assume success when we replicate an existing leaf

The logic in couch_db:prep_and_validate_replicated_updates/5 was broken
if we attempt to replicate a leaf that exists. The function has
explicitly commented logic that if we attempt to replicate an internal
node of the revision tree that we just ignore it and assume success.
While the logic matched the comment, what the comment should say is that
if we replicate an existing revision then we should treat is as a
successful no-op.

The way that the error manifests with the old logic is that we would try
and run make_first_doc_on_disk which reads through all previous
revisions on the edit path to find the first available revision.
Although if we had compacted then none would exist so that the
make_first_doc_on_disk function would return nil and attempt to use that
where we wanted a #doc{} in couch_doc:merge_stubs/2.

BugzId: 15562