The table's PeerDown visits every prefix under the write lock, which
takes over a second on a full table. The prefix handler called it
inline, so its queue filled and the streamer dropped updates.
PeerDown now holds the lock for 10,000 prefixes at a time, then lets
the lock go for as long again. The handler runs PeerDown on a separate
goroutine, one peer at a time. The handler holds back the down peer's
later messages until that PeerDown returns. Then the handler applies
them in order, one at a time, and takes new messages in between. It
holds back at most 20,000 messages. Stop waits for all of this before
its last flush.
Model: opus-5-5
