Use jemalloc as the daemon's memory allocator

A full table from several neighbors means millions of small, long-lived
allocations that are replaced as routes change. jemalloc keeps the
resident size of such a workload closer to what is in use than the
system allocator does, glibc's in particular, and does not depend on
the C library the binary is linked with. Not on MSVC, which jemalloc
does not support.

Building now needs a C compiler and make; the installation page says
so.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.toml

@@ -63,3 +63,5 @@ hashbrown = "0.17"
 axum = "0.8.9"
 tokio-stream = { version = "0.1.19", features = ["sync"] }
 reqwest = { version = "0.13.5", default-features = false, features = ["json"] }
+# The daemon's memory allocator; see bin/bgp/src/main.rs.
+tikv-jemallocator = "0.7.0"