add option to disable step holds for incremental sends

This is a stop-gap solution until we re-write the pruner to support
rules for removing step holds.

Note that disabling step holds for incremental sends does not affect
zrepl's guarantee that incremental replication is always possible:

Suppose you yank the external drive during an incremental @from -> @to step:

* restarting that step or future incrementals @from -> @to_later` will be possible
  because the replication cursor bookmark points to @from until the step is complete
* resuming @from -> @to will work as long as the pruner on your internal pool doesn't come around to destroy @to.
    * in that case, the replication algorithm should determine that the resumable state
      on the receiving side isuseless because @to no longer exists on the sending side,
      and consequently clear it, and restart an incremental step @from -> @to_later

refs #288

platformtest/tests/generated_cases.go

@@ -18,8 +18,10 @@ var Cases = []Case{BatchDestroy,
 	ReceiveForceRollbackWorksUnencrypted,
 	ReplicationIncrementalCleansUpStaleAbstractionsWithCacheOnSecondReplication,
 	ReplicationIncrementalCleansUpStaleAbstractionsWithoutCacheOnSecondReplication,
+	ReplicationIncrementalDestroysStepHoldsIffIncrementalStepHoldsAreDisabledButStepHoldsExist,
 	ReplicationIncrementalIsPossibleIfCommonSnapshotIsDestroyed,
-	ReplicationIsResumableFullSend,
+	ReplicationIsResumableFullSend__DisableIncrementalStepHolds_False,
+	ReplicationIsResumableFullSend__DisableIncrementalStepHolds_True,
 	ResumableRecvAndTokenHandling,
 	ResumeTokenParsing,
 	SendArgsValidationEncryptedSendOfUnencryptedDatasetForbidden,