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.

zfs/zfs.go

@@ -490,7 +490,7 @@ func ZFSGet(fs *DatasetPath, props []string) (*ZFSProperties, error) {
 	return zfsGet(fs.ToString(), props)
 }
 
-var zfsGetDatasetDoesNotExistRegexp = regexp.MustCompile(`^cannot open '(\S+)': dataset does not exist`)
+var zfsGetDatasetDoesNotExistRegexp = regexp.MustCompile(`^cannot open '(\S+)': (dataset does not exist|no such pool or dataset)`)
 
 type DatasetDoesNotExist struct {
 	Path string