Keep one entry per AS in the AS handler's batch (closes #217)

HandleMessage added one entry to the batch for every AS of every path,
and waited for the write in progress once the batch held 300,000. In a
burst of the feed its queue then filled, and the streamer dropped
messages.

The batch is now a map from AS number to the earliest and the latest
time of its updates. It holds one entry per AS, so it no longer grows
with the feed's rate. HandleMessage never waits for a write.
asnBatchSize now counts ASes. Stop logs the entries of the batch being
written and of the next batch under "batch_entries". An AS in both
counts twice.

Model: opus-5-5

TODO.md

@@ -35,6 +35,11 @@ tested.
 
 # Completed Steps
 
+- 2026-11-02: the AS handler never waits for a database write. Its batch holds
+  one entry per AS, with the earliest and the latest time of its updates. So the
+  batch no longer grows with the feed's rate. It used to hold one entry for each
+  AS of each path and to wait once it held 300,000. In a burst of the feed its
+  queue then filled, and the streamer dropped messages for it (closes #217)
 - 2026-10-27: the tests of `internal/routewatch` no longer fail on a loaded
   host. Five of them gave `HandleMessage` two seconds to apply a full prefix
   batch, which under the race detector can take longer. A test now waits up to