daemon/active: implement watchdog to handle stuck replication / pruners

ActiveSide.do() can only run sequentially, i.e. we cannot run
replication and pruning in parallel. Why?

* go-streamrpc only allows one active request at a time
(this is bad design and should be fixed at some point)
* replication and pruning are implemented independently, but work on the
same resources (snapshots)

A: pruning might destroy a snapshot that is planned to be replicated
B: replication might replicate snapshots that should be pruned

We do not have any resource management / locking for A and B, but we
have a use case where users don't want their machine fill up with
snapshots if replication does not work.
That means we _have_ to run the pruners.

A further complication is that we cannot just cancel the replication
context after a timeout and move on to the pruner: it could be initial
replication and we don't know how long it will take.
(And we don't have resumable send & recv yet).

With the previous commits, we can implement the watchdog using context
cancellation.
Note that the 'MadeProgress()' calls can only be placed right before
non-error state transition. Otherwise, we could end up in a live-lock.

client/status.go

@@ -274,8 +274,7 @@ func (t *tui) renderReplicationReport(rep *replication.Report) {
 		t.printf("Problem: %s", rep.Problem)
 		t.newline()
 	}
-	if rep.SleepUntil.After(time.Now()) &&
-		state & ^(replication.PermanentError|replication.Completed) != 0 {
+	if rep.SleepUntil.After(time.Now()) && !state.IsTerminal() {
 		t.printf("Sleeping until %s (%s left)\n", rep.SleepUntil, rep.SleepUntil.Sub(time.Now()))
 	}