bgp: jemalloc's background thread on Linux only

The last change built jemalloc with its background thread on, so that
freed pages go back to the kernel on time. jemalloc has that thread on
Linux only: on macOS every start of the daemon, and every test that
starts one, printed "option background_thread currently supports
pthread only".

The feature now comes from a Linux-only dependency entry of the daemon
crate, so the macOS build asks for nothing jemalloc cannot do. The
Linux build is as before: measured in the VM with four peers and a
collector, resident memory falls after the load without any setting
in the environment.

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

Cargo.toml

@@ -65,7 +65,7 @@ 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. With its
 # statistics, which the status page and the metrics show.
-tikv-jemallocator = { version = "0.7.0", features = ["stats", "background_threads"] }
+tikv-jemallocator = { version = "0.7.0", features = ["stats"] }
 tikv-jemalloc-ctl = { version = "0.7.0", features = ["stats"] }
 
 # `cargo build --profile profiling`: the release build, with the line tables