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

replication/logic/pdu/pdu_extras.go

@@ -83,3 +83,10 @@ func (v *FilesystemVersion) ZFSFilesystemVersion() (*zfs.FilesystemVersion, erro
 		Creation:  ct,
 	}, nil
 }
+
+func ReplicationConfigProtectionWithKind(both ReplicationGuaranteeKind) *ReplicationConfigProtection {
+	return &ReplicationConfigProtection{
+		Initial:     both,
+		Incremental: both,
+	}
+}