ReplicatedPG: enforce write ordering on rollback

ReplicatedPG: enforce write ordering on rollback

Previously, rollback ops could reorder w.r.t other writes due to waiting
on degraded snaps other than head.  To fix that, we'll introduce a new
map tracking objects blocked on degraded snaps.  A particular object can
only be blocked on one snap at a time (subsequent writes won't get far
enough to add another entry).

It might have been possible use the blocked_by machinery for this, but
it requires that the object have an extant obc, which we may not
have for a missing object.  Also, that machinery exists primarily to
support clone_range, which I hope to remove soon.

Signed-off-by: Samuel Just <sjust@redhat.com>