Start no prefix flush after the stop deadline (closes #188)

When a write ran past the stop deadline, a full batch's wake-up or a tick
could wait beside Stop's context. The flush goroutine's select picked one of
them at random, so about half the time it wrote that batch after the
deadline. That was the intermittent failure of
TestPrefixHandlerSkipsItsLastWriteAfterTheStopDeadline. The flush goroutine
now takes a waiting stop first. A new test hands over a full batch while a
write is held; it fails about half its runs on the old code.

TestPrefixHandlerSkipsItsLastWriteAfterTheStopDeadline and
TestPrefixHandlerStopLogsTheTableChangesLeftUnwritten also failed when the
flush timer fired between two of their steps. Both now give the handler a
timer interval longer than the test.

Model: opus-5-5

internal/routewatch/prefixhandler_internal_test.go

@@ -980,6 +980,11 @@ func TestPrefixHandlerStopLogsNothingAfterADeleteThatSucceeds(t *testing.T) {
 	}
 }
 
+// longerThanAnyTest is a flush timer interval that no test lasts. A test whose
+// prefix handler must flush only when its batch is full and on Stop gives it to
+// newPrefixHandler.
+const longerThanAnyTest = time.Hour
+
 // prefixHandlerWithoutTimer returns a prefix handler that writes to store and
 // applies the updates to an empty routing table. It is built without
 // NewPrefixHandler's flush goroutine, so the test flushes itself.