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/pdu/pdu.proto

@@ -98,21 +98,21 @@ message DestroySnapshotsRes {
     repeated DestroySnapshotRes Results = 1;
 }
 
-message SnapshotReplicationStatusReq {
+message ReplicationCursorReq {
     string Filesystem = 1;
-    string Snapshot = 2;
-    enum Op {
-        Get = 0;
-        SetReplicated = 1;
+    message GetOp {}
+    message SetOp {
+        string Snapshot = 2;
+    }
+    oneof op {
+        GetOp get = 2;
+        SetOp set = 3;
     }
-    Op op = 3;
 }
 
-message SnapshotReplicationStatusRes {
-    enum Status {
-        Nonexistent = 0;
-        NotReplicated = 1;
-        Replicated = 2;
+message ReplicationCursorRes {
+    oneof Result {
+        uint64 Guid = 1;
+        string Error = 2;
     }
-    Status status = 1;
 }