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.proto

@@ -61,6 +61,25 @@ message SendReq {
   Tri Encrypted = 5;
 
   bool DryRun = 6;
+
+  ReplicationConfig ReplicationConfig = 7;
+}
+
+message ReplicationConfig {
+  ReplicationConfigProtection protection = 1;
+}
+
+
+message ReplicationConfigProtection {
+  ReplicationGuaranteeKind Initial = 1;
+  ReplicationGuaranteeKind Incremental = 2;
+}
+
+enum ReplicationGuaranteeKind {
+  GuaranteeInvalid = 0;
+  GuaranteeResumability = 1;
+  GuaranteeIncrementalReplication = 2;
+  GuaranteeNothing = 3; 
 }
 
 message Property {
@@ -93,6 +112,8 @@ message ReceiveReq {
   // If true, the receiver should clear the resume token before performing the
   // zfs recv of the stream in the request
   bool ClearResumeToken = 3;
+
+  ReplicationConfig ReplicationConfig = 4;
 }
 
 message ReceiveRes {}