replication & pruning: ditch replicated-property, use bookmark as cursor instead

A bookmark with a well-known name is used to track which version was
last successfully received by the receiver.
The createtxg that can be retrieved from the bookmark using `zfs get` is
used to set the Replicated attribute of each snap on the sender:
If the snap's CreateTXG > the cursor's, it is not yet replicated,
otherwise it has been.

There is an optional config option to change the behvior to
`CreateTXG >= the cursor's`, and the implementation defaults to that.

The reason: While things work just fine with `CreateTXG > the cursor's`,
ZFS does not provide size estimates in a `zfs send` dry run
(see acd2418).
However, to enable the use case of keeping the snapshot only around for
the replication, the config flag exists.

replication/internal/diff/diff.go

@@ -59,10 +59,9 @@ func IncrementalPath(receiver, sender []*FilesystemVersion) (incPath []*Filesyst
 
 	for mrcaRcv >= 0 && mrcaSnd >= 0 {
 		if receiver[mrcaRcv].Guid == sender[mrcaSnd].Guid {
-			if mrcaSnd-1 >= 0 && sender[mrcaSnd-1].Guid == sender[mrcaSnd].Guid && sender[mrcaSnd-1].Type == FilesystemVersion_Bookmark {
-				// prefer bookmarks over snapshots as the snapshot might go away sooner
-				mrcaSnd -= 1
-			}
+			// Since we arrive from the end of the array, and because we defined bookmark < snapshot,
+			// this condition will match snapshot first, which is what we want because it gives us
+			// size estimation
 			break
 		}
 		receiverCreation, err := receiver[mrcaRcv].CreationAsTime()