replication + endpoint: replication guarantees: guarantee_{resumability,incremental,nothing}

This commit

- adds a configuration in which no step holds, replication cursors, etc. are created
- removes the send.step_holds.disable_incremental setting
- creates a new config option `replication` for active-side jobs
- adds the replication.protection.{initial,incremental} settings, each
  of which can have values
    - `guarantee_resumability`
    - `guarantee_incremental`
    - `guarantee_nothing`
  (refer to docs/configuration/replication.rst for semantics)

The `replication` config from an active side is sent to both endpoint.Sender and endpoint.Receiver
for each replication step. Sender and Receiver then act accordingly.

For `guarantee_incremental`, we add the new `tentative-replication-cursor` abstraction.
The necessity for that abstraction is outlined in https://github.com/zrepl/zrepl/issues/340.

fixes https://github.com/zrepl/zrepl/issues/340

platformtest/tests/replicationCursor.go

@@ -22,6 +22,7 @@ func CreateReplicationCursor(ctx *platformtest.Context) {
 		R  zfs bookmark "${ROOTDS}/foo bar@2 with space" "${ROOTDS}/foo bar#2 with space"
 		+  "foo bar@3 with space"
 		R  zfs bookmark "${ROOTDS}/foo bar@3 with space" "${ROOTDS}/foo bar#3 with space"
+		-  "foo bar@3 with space"
 	`)
 
 	jobid := endpoint.MustMakeJobID("zreplplatformtest")
@@ -42,6 +43,7 @@ func CreateReplicationCursor(ctx *platformtest.Context) {
 
 	snap := fsversion(ctx, fs, "@1 with space")
 	book := fsversion(ctx, fs, "#1 with space")
+	book3 := fsversion(ctx, fs, "#3 with space")
 
 	// create first cursor
 	cursorOfSnap, err := endpoint.CreateReplicationCursor(ctx, fs, snap, jobid)
@@ -49,8 +51,11 @@ func CreateReplicationCursor(ctx *platformtest.Context) {
 	// check CreateReplicationCursor is idempotent (for snapshot target)
 	cursorOfSnapIdemp, err := endpoint.CreateReplicationCursor(ctx, fs, snap, jobid)
 	checkCreateCursor(err, cursorOfSnap, snap)
+	// check CreateReplicationCursor is idempotent (for bookmark target of snapshot)
+	cursorOfBook, err := endpoint.CreateReplicationCursor(ctx, fs, book, jobid)
+	checkCreateCursor(err, cursorOfBook, snap)
 	// ... for target = non-cursor bookmark
-	_, err = endpoint.CreateReplicationCursor(ctx, fs, book, jobid)
+	_, err = endpoint.CreateReplicationCursor(ctx, fs, book3, jobid)
 	assert.Equal(ctx, zfs.ErrBookmarkCloningNotSupported, err)
 	// ... for target = replication cursor bookmark to be created
 	cursorOfCursor, err := endpoint.CreateReplicationCursor(ctx, fs, cursorOfSnapIdemp.GetFilesystemVersion(), jobid)