rib: give a row room for every peer's route at its first growth

A row's routes are in a vector that grew by one route at a time, so with
four peers sending full tables every row's vector was reallocated three
times as their routes arrived, and the reallocations were 7% of the
daemon's CPU time in a profile of a four-peer load.

This change has the first growth of a row take it to as many routes as
the table has peers, at most 8 (`FIRST_ROOM`), since a prefix that one
full table has, the others nearly always have too; after that it grows
by a quarter as before. A table with one peer never grows a row and is
unchanged. A route server's clients do not all announce every prefix,
hence the cap: a row of a prefix that fewer peers announce has at most
seven spare slots of 16 bytes.

Measured in the Linux VM with four peers sending the synthetic table of
1.23 million prefixes: the kernel is in step 1.9 s after the first route
instead of 2.2 s, the daemon's CPU time over the load is 5% lower, and
it is 604 MiB resident instead of about 630, since there are no
reallocation leftovers. With one peer nothing changes. Memory per route
in the benchmarks, allocations counted: 77 bytes before and after with
the synthetic table from four peers; 64 before and 65 after with the
RIPE RIS dump's eight largest peers, whose load went from 2.8 s to
1.8 s on one thread, the allocation counting making every reallocation
dearer than it is in the daemon.

Also tried and not adopted, with the new `JEMALLOC_CONF` setting of the
profiling script: transparent huge pages for the allocator
(`thp:always,metadata_thp:always`). Over three pairs of runs with one
peer the table arrived in 0.66, 1.17 and 0.70 s with them against 0.87,
0.75 and 0.87 s without, the CPU time was within the noise, and the
daemon was 60 to 100 MiB larger. An arena for the routes would not help
either: jemalloc's size classes already give rows exact 16, 32, 48 and
64 byte allocations, and the cost was the number of reallocations, not
the allocator.

Testing: the table tests, including the one that grows a row past
65,535 routes, pass; the kernel tests and the interoperability lab pass.

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

scripts/profile/full-table.sh

@@ -104,7 +104,9 @@ if [ "$kernel" = 1 ]; then
     echo 'kernel { stale_seconds: 0 }' >> "$out/daemon.textpb"
 fi
 
-run "$daemon_bin" "$out/daemon.textpb" --level warn > "$out/daemon.log" 2>&1 &
+# `JEMALLOC_CONF`, if set, is jemalloc's MALLOC_CONF for the daemon:
+# "thp:always", say. The symbols are prefixed, hence the variable's name.
+run env _RJEM_MALLOC_CONF="${JEMALLOC_CONF:-}" "$daemon_bin" "$out/daemon.textpb" --level warn > "$out/daemon.log" 2>&1 &
 for _ in $(seq 100); do
     run curl -sf http://127.0.0.1:8080/api/status > /dev/null 2>&1 && break
     sleep 0.1