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

config/config.go

@@ -76,7 +76,12 @@ type SnapJob struct {
 }
 
 type SendOptions struct {
-	Encrypted bool `yaml:"encrypted"`
+	Encrypted bool                 `yaml:"encrypted"`
+	StepHolds SendOptionsStepHolds `yaml:"step_holds,optional"`
+}
+
+type SendOptionsStepHolds struct {
+	DisableIncremental bool `yaml:"disable_incremental,optional"`
 }
 
 var _ yaml.Defaulter = (*SendOptions)(nil)