ReplicatedPG: use our already-found iterator instead of going back into map

We have an iterator pointing at the element we want; no need to search
the map again in order to grab the element or remove it.

Signed-off-by: Greg Farnum <greg@inktank.com>
ReplicatedPG: remove useless pcop param from start_copy

Signed-off-by: Greg Farnum <greg@inktank.com>
ReplicatedPG: remove unused CopyOp::waiting member, rename function for accuracy

We never placed any ops here, and so we weren't ever requeuing any ops.

Signed-off-by: Greg Farnum <greg@inktank.com>