session: write messages in batches, with TCP_NODELAY

A session wrote every message to its socket as it came: one write and,
with Nagle's algorithm, mostly one segment for each UPDATE, a hundred or
two octets. On the IPv6 cross-connect between the two routers, whose
round trip is 50 microseconds, a million routes took 42 seconds to go
out: the status page's new TCP row showed the connection busy for that
long with neither window nor buffer limiting it, a congestion window
stuck at ten segments, and a smoothed round-trip time of 18 ms. That is
the receiver's delayed acknowledgements, 40 ms at a time, on a stream
of small segments, holding the sender to a few segments a round trip.

Now a session gathers what is waiting for it, up to the outbound queue's
length, in one wake, encodes the messages into one buffer, and writes
the buffer when it reaches 64 KiB or when the actions of the wake are
done; the socket has TCP_NODELAY, so a batch's last, partial segment is
not held back for the acknowledgement of the one before. The
NOTIFICATION that ends a session, including one sent after a panic, is
flushed on its own.

Measured in the Linux VM with four peers' tables going to a collector
over a veth pair, 151 MiB: 121,850 segments instead of 313,773, the
connection busy 0.8 s instead of 2.0 s, and the congestion window grown
to 47 segments instead of 13. Over loopback the transit benchmark is
unchanged at 2.5 s for the second neighbor to have the table: there the
receiver's parsing sets the pace, not the segments.

Testing: the session tests, including the one that sends a NOTIFICATION
after a panic; the integration tests; the kernel tests and every lab
phase.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

scripts/profile/full-table.sh

@@ -195,6 +195,9 @@ if [ "$collector" = 1 ]; then
         last=$now_held
     done
     echo "collector: $(advertised) routes advertised after $(since "$start") s"
+    collector_addr=$(status peers '.[] | select(.name == "collector") | .peer_addr')
+    run curl -sf "http://127.0.0.1:8080/api/tcp?peer=$collector_addr" \
+        | jq -r '.connection | "collector tcp: busy \(.busy_ms / 1000 | . * 10 | round / 10) s, rwnd-limited \(.limited_by_receive_window_share * 100 | round)%, rtt \(.rtt_ms) ms, cwnd \(.cwnd_segments), sent \(.bytes_sent / 1048576 | round) MiB in \(.segments_out) segments"'
 fi
 memory_report() {
     rss=$(awk '/VmRSS/ { printf "%.0f", $2 / 1024 }' "/proc/$daemon_pid/status")