Make DnsHandle::send &self instead of &mut self

[ibigbug]

following #1933

making the trait non mut so usage of it doesn't need Mutex

[ibigbug]

cc @djc

[bluejekyll]

wow, I'm amazed this required so few changes. This has probably been &mut since the first non-async implementations. This should definitely make the interface simpler for folks to use. I'll review in more detail once the tests are passing.

[ibigbug]

thank you @bluejekyll for the awesome project it's definetely help me a lot in building my project and hope i can contribute some too

[ibigbug]

seems coverage is failing for some reason

[bluejekyll]

Coverage is for reference and to draw attention to areas where we might not have great coverage so we should take more care on the review. So don't worry about those.

@djc, I'm ok with this change, though it will mean that with out this being mut, if for some reason we need it to be in the future, it will be a harder and bigger breaking change. I don't know whey that would be, but that is the only negative I see to this.

[djc]

LGTM, modulo one tiny nit.

[djc]

Thanks for fixing that up -- and should have squashed this during merge, sorry for messing that up.