bench: two routers exchanging their tables with ADD-PATH

The full-table benchmark measures one router taking in tables from its
peers. What two routers of one AS do with each other, each sending the
other every path it has (ADD-PATH both ways with all_paths, as a mesh
of routers that each hold the network's whole view is configured), was
measured nowhere: the exporter following the table's journal while the
sessions fill the table, at the size of the Internet. In production
that exchange took 70 seconds for a table that arrives from the
Internet in 20.

A new benchmark, mesh, starts two servers in one AS with distinct
router IDs, peers them over IPv6 loopback with ADD-PATH both ways and
all_paths, and feeds the first from four synthetic peers and the second
from two over IPv4 loopback (BENCH_PEERS, BENCH_PEERS_B,
BENCH_SCALE_DOWN). It prints how long each router takes to hold its
own peers' routes and then the other's, with the rate of the latter.
BENCH_DEADLINE prints the sessions' state if a router is not there in
time.

The benchmarks now allocate with jemalloc, as the daemon does, except
when memory is being counted: the C library's allocator put its own
locks in the profiles. Two routers on one host have each other's
addresses as next hops, loopback addresses, which the import check
refuses as it refuses a next hop that is this machine; the permissive
import policy, which only tests use and no configuration can ask for,
now takes them.

Testing: the next-hop test covers both policies; the benchmark runs on
macOS and in the Linux VM under perf.

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

Cargo.lock

@@ -182,6 +182,7 @@ dependencies = [
  "bytes",
  "flate2",
  "stats_alloc",
+ "tikv-jemallocator",
  "tokio",
 ]