Start no AS, peer or peering write after the stop deadline (closes #189)

A write can run past the stop deadline while a full batch's wake-up or a
tick waits beside Stop's context. The select picked one at random, so about
half the time the AS, peer or peering handler started another write after
the deadline. Each loop now takes a waiting stop first, as the prefix
handler's does. The prefix, AS and peer handlers' flush loops were
near-copies, so they are now one function, flushLoop. A new test for each
handler fails about half its runs on the old code.

The AS and peer handler tests give the flush timer an interval longer than
the test. A timer flush during the fill can no longer hang
TestPeerHandlerWaitsAtTheMaximumBatchSize.

Model: opus-5-5

internal/routewatch/prefixhandler_internal_test.go

@@ -981,8 +981,8 @@ 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.
+// prefix, AS or peer handler must flush only when its batch is full and on Stop
+// gives it to newPrefixHandler, newASHandler or newPeerHandler.
 const longerThanAnyTest = time.Hour
 
 // prefixHandlerWithoutTimer returns a prefix handler that writes to store and