replication: report a filesystem is active vs. blocked on something

- `BlockedOn` prop in JSON report
- Bring back the `*` in front of the filesystem report as an activity indicator.

fixes https://github.com/zrepl/zrepl/issues/505

replication/report/replication_report.go

@@ -62,11 +62,23 @@ const (
 	FilesystemDone            FilesystemState = "done"
 )
 
+type FsBlockedOn string
+
+const (
+	FsBlockedOnNothing           FsBlockedOn = "nothing"
+	FsBlockedOnPlanningStepQueue FsBlockedOn = "plan-queue"
+	FsBlockedOnParentInitialRepl FsBlockedOn = "parent-initial-repl"
+	FsBlockedOnReplStepQueue     FsBlockedOn = "repl-queue"
+)
+
 type FilesystemReport struct {
 	Info *FilesystemInfo
 
 	State FilesystemState
 
+	// Always valid.
+	BlockedOn FsBlockedOn
+
 	// Valid in State = FilesystemPlanningErrored
 	PlanError *TimedError
 	// Valid in State = FilesystemSteppingErrored