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/app.go

@@ -2,7 +2,9 @@
 // running instance of the application and contains pointers to its core
 // dependencies, and is responsible for initialization. It also contains the
 // ASN WHOIS fetcher for background updates and the database maintainer for
-// background maintenance tasks.
+// background maintenance tasks. The package also holds the handlers that
+// write the feed's updates to the database in batches, and the flush loop
+// that the AS, peer and prefix handlers share.
 package routewatch
 
 import (