tools.lazy-store: tokio and reqwest instead of ureq and threads

The I/O runs on a tokio runtime now:

- HTTP via reqwest (rustls on aws-lc, HTTP/2, bundled webpki roots as
before) instead of ureq. Instead of our own caching ureq::Resolver the
cache hosts are resolved once at startup and pinned with
resolve_to_addrs, which keeps getaddrinfo() (and NSS modules from the
store) out of requests once /nix/store is mounted. The retry loop
stays: reqwest-middleware/retry would not save code, as body read
errors are retried too.
- Decompression via async-compression (zstd, xz, bzip2, gzip, brotli,
concatenated members like Nix) instead of zstd/xz2/bzip2/flate2/
brotli-decompressor; the magic sniffing for narinfo/.ls stays.
- The closure walk and the .ls listings run with buffer_unordered
instead of par_map threads.
- FUSE workers stay threads; open() fetches via Handle::block_on, and
concurrent opens of a path wait on a tokio Mutex.
- The compressed size is counted with tokio-util's InspectReader instead
of our CountingReader. The NAR is still unpacked by our reader for now,
through SyncIoBridge.