osd/ReplicatedPG: do not assert on clone_size mismatch

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: improve clone vs head checking

- notice when we are missing a clone (that isn't at the end of the list)
- notice when we are missing a clone on the last object in the scrub map
- do not assert when we are missing a clone

There is still more we could do to improve this (like noticing one missing
clone but still checking the others), but we'll leave that aside for just
a moment...

Signed-off-by: Sage Weil <sage@inktank.com>
osd/ReplicatedPG: tolerate missing clones in cache pools

A few cases:

- As we are working through the list, if we see a clone that is lower than
  the next one we were expecting, we should be able to skip them.
- If we see a head, we can skip all of the rest of the clones.
- If we get to the end and next_clone was set, we can ignore it.

Signed-off-by: Sage Weil <sage@inktank.com>