ReplicatedPG: remove old-head optization from push_to_replica

This optimization allowed the primary to push a clone as a single push in the
case that the head object on the replica is old and happens to be at the same
version as the clone.  In general, using head in clone_subsets is tricky since
we might be writing to head during the push.  calc_clone_subsets does not
consider head (probably for this reason).  Handling the clone from head case
properly would require blocking writes on head in the interim which is probably
a bad trade off anyway.

Because the old-head optimization only comes into play if the replica's state
happens to fall on the last write to head prior to the snap that caused the
clone in question, it's not worth the complexity.

Fixes: #3698
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>