Add command-line option "dns-cache-size" for trust-dns

[zedifen]

See also #1215

This PR adds a command-line option --dns-cache-size to sslocal, ssserver and ssmanager, allowing users to configure the cache_size of ResolverOpts in trust-dns.

I'm not sure if the current solution is appropriate, i.e. to pass dns_cache_size all the way to create_resolver. How about I pass the dns_cache_size through a ResolverOpts?

[zonyitoo]

Please check the clippy logs and fix those warnings.

[zedifen]

Improve code

[fernvenue]

Maybe mention this change in README?

[zonyitoo]

Maybe mention this change in README?

Sounds great!

[fernvenue]

Tested, works great :)

[zedifen]

I'm going to rebase this branch 😊

Update: rebased.

[zedifen]

Oh, by the way, currently the musl cross-build would fail on stable rust toolchain with recent ziglang versions. Moving to rust nightly would fix it.

Or, the rust is going to stabilize the new musl version shipped with the toolchain in 1.71. See https://blog.rust-lang.org/2023/05/09/Updating-musl-targets.html

One may refer to the PR in my fork of the repository. https://github.com/zedifen/shadowsocks-rust/pull/4

[zonyitoo]

I am still waiting trust-dns to stablize its new version, so it is Ok to wait 1.71 until making the next release.