PGTransaction,ReplicatedPG: clarify handling of noop operations

PGTransaction,ReplicatedPG: clarify handling of noop operations

The offending transaction was [call rbd.copyup,delete] on a non-existent
object.  PGTransaction incorrectly ended up with Create and delete_first
causing a transaction beginning with trying to collection_move_rename a
non-existent head object.  In fact, if we delete an object which the
transaction currently claims to be creating, the transaction should show
as empty (for that object).  Rather than going through the normal write
pipeline for that case, let's just record a 0 error code in the log and
call it a day.  That way, the transaction generating code only needs to
worry about about updates to actual objects.

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