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/generated_cases.go

@@ -20,8 +20,11 @@ var Cases = []Case{BatchDestroy,
 	ReplicationIncrementalCleansUpStaleAbstractionsWithoutCacheOnSecondReplication,
 	ReplicationIncrementalDestroysStepHoldsIffIncrementalStepHoldsAreDisabledButStepHoldsExist,
 	ReplicationIncrementalIsPossibleIfCommonSnapshotIsDestroyed,
-	ReplicationIsResumableFullSend__DisableIncrementalStepHolds_False,
-	ReplicationIsResumableFullSend__DisableIncrementalStepHolds_True,
+	ReplicationIsResumableFullSend__both_GuaranteeResumability,
+	ReplicationIsResumableFullSend__initial_GuaranteeIncrementalReplication_incremental_GuaranteeIncrementalReplication,
+	ReplicationIsResumableFullSend__initial_GuaranteeResumability_incremental_GuaranteeIncrementalReplication,
+	ReplicationStepCompletedLostBehavior__GuaranteeIncrementalReplication,
+	ReplicationStepCompletedLostBehavior__GuaranteeResumability,
 	ResumableRecvAndTokenHandling,
 	ResumeTokenParsing,
 	SendArgsValidationEncryptedSendOfUnencryptedDatasetForbidden,