Make a timed batch write on every tick (closes #192)

The prefix, AS and peer handlers skipped a timed write unless one
interval had passed since the last write ended. The next tick came a
little less than one interval after that, so every other tick was
skipped. Each tick now writes. A full batch's write restarts the timer,
so the next timed write comes one interval after that write began.
Comparing clock readings would not do: two ticks one interval apart can
read a hair under it. flushBatch no longer reports whether it wrote a
batch, since nothing reads that. A new test runs the loop on a fake
clock and fails on the old code.

Model: opus-5-5

internal/routewatch/peerhandler.go

@@ -200,11 +200,11 @@ func (h *PeerHandler) takeBatch() []peerUpdate {
 }
 
 // flushBatch takes the pending batch and writes it to the database, without
-// holding the mutex during the write. It reports whether there was a batch.
-func (h *PeerHandler) flushBatch() bool {
+// holding the mutex during the write.
+func (h *PeerHandler) flushBatch() {
 	batch := h.takeBatch()
 	if len(batch) == 0 {
-		return false
+		return
 	}
 
 	// The earliest and the latest time the batch saw each peer, with the peer
@@ -234,6 +234,4 @@ func (h *PeerHandler) flushBatch() bool {
 	h.mu.Lock()
 	h.writing = 0
 	h.mu.Unlock()
-
-	return true
 }