snapshotting: support 'periodic' and 'manual' mode

1. Change config format to support multiple types
   of snapshotting modes.
2. Implement a hacky way to support periodic or completely
   manual snaphots.

In manual mode, the user has to trigger replication using the wakeup
mechanism after they took snapshots using their own tooling.

As indicated by the comment, a more general solution would be desirable,
but we want to get the release out and 'manual' mode is a feature that
some people requested...

config/config.go

@@ -38,7 +38,7 @@ type PassiveJob struct {
 
 type PushJob struct {
 	ActiveJob `yaml:",inline"`
-	Snapshotting Snapshotting          `yaml:"snapshotting"`
+	Snapshotting SnapshottingEnum          `yaml:"snapshotting"`
 	Filesystems FilesystemsFilter `yaml:"filesystems"`
 }
 
@@ -55,15 +55,24 @@ type SinkJob struct {
 
 type SourceJob struct {
 	PassiveJob `yaml:",inline"`
-	Snapshotting Snapshotting      `yaml:"snapshotting"`
+	Snapshotting SnapshottingEnum      `yaml:"snapshotting"`
 	Filesystems FilesystemsFilter `yaml:"filesystems"`
 }
 
 type FilesystemsFilter map[string]bool
 
-type Snapshotting struct {
-	SnapshotPrefix string        `yaml:"snapshot_prefix"`
-	Interval       time.Duration `yaml:"interval,positive"`
+type SnapshottingEnum struct {
+	Ret interface{}
+}
+
+type SnapshottingPeriodic struct {
+	Type string		`yaml:"type"`
+	Prefix string	`yaml:"prefix"`
+	Interval time.Duration `yaml:"interval,positive"`
+}
+
+type SnapshottingManual struct {
+	Type string `yaml:"type"`
 }
 
 type PruningSenderReceiver struct {
@@ -346,6 +355,14 @@ func (t *PruningEnum) UnmarshalYAML(u func(interface{}, bool) error) (err error)
 	return
 }
 
+func (t *SnapshottingEnum) UnmarshalYAML(u func(interface{}, bool) error) (err error) {
+	t.Ret, err = enumUnmarshal(u, map[string]interface{}{
+		"periodic": &SnapshottingPeriodic{},
+		"manual": &SnapshottingManual{},
+	})
+	return
+}
+
 func (t *LoggingOutletEnum) UnmarshalYAML(u func(interface{}, bool) error) (err error) {
 	t.Ret, err = enumUnmarshal(u, map[string]interface{}{
 		"stdout": &StdoutLoggingOutlet{},