Write the routing table every second and read it at startup (part of #122)

The prefix handler applies each update to the routing table in memory as
it handles it, sorted into announcements and withdrawals as for the live
route tables. Each flush writes the table's changes first. A failed write
puts its changes and new peers back in the table, so the next write takes
them with each pair's content then. Stop writes the last changes, and
logs those left when its deadline passes.

Startup reads the table before the streamer connects, and logs its pairs,
peers and time. The per-peer counts are a slice, so the read takes about
a third less time. The health check's start period is 60 seconds, so a
full table's read fits before upaas looks.

Model: opus-5-5

Dockerfile

@@ -139,8 +139,13 @@ EXPOSE 8080
 
 COPY ./entrypoint.sh /entrypoint.sh
 
-# Health check using the health endpoint, on the port PORT names
-HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+# Health check using the health endpoint, on the port PORT names. During the
+# start period, Docker 25 and later checks every 5 seconds, and the first pass
+# makes the container healthy. After it, Docker checks every 30 seconds. The
+# start period covers the 60 seconds after which upaas reads the health
+# (README "Running under upaas"). Startup reads the routing table first,
+# which takes about 25 seconds when the table is full.
+HEALTHCHECK --interval=30s --timeout=5s --start-period=60s --retries=3 \
     CMD curl -sf "http://localhost:${PORT:-8080}/.well-known/healthcheck.json" || exit 1
 
 ENTRYPOINT ["/bin/bash", "/entrypoint.sh" ]