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>

docs/content/docs/development/testing.md

@@ -214,6 +214,7 @@ environment variables that control the run.
 | `HOLD_SECS` | How long the sessions stay open after the table is sent | 30 |
 | `FREQ` | The sampling frequency in hertz | 1999 |
 | `OUT` | The directory for the results | `~/profile` |
+| `JEMALLOC_CONF` | jemalloc options for the daemon (its `MALLOC_CONF`), for example `thp:always` | none |
 
 When run through `scripts/linux-vm.sh`, pass the variables with `env`,
 because the virtual machine does not receive the host's environment: