Drop `tokio/macros` dependency in `lightning-net-tokio`, fix MSRV

Drop `tokio/macros` dependency in `lightning-net-tokio`, fix MSRV

The `tokio` `macros` feature depends on `proc-macro2`, which
recently broke its MSRV in a patch version. Such crates aren't
reasonable for us to have as dependencies, so instead we replace
the one trivial use we have of `tokio::select!()` with our own
manual future.