Refactor the Handler traits

[ricott1]

Follow up to #191.

• Remove the owned pattern with tuple return for the Handlers traits in favour of &mut references.

[ricott1]

I was also thinking about moving run and run_on_socket to the Server trait, I will do that in a separate commit, what do you think?

[lowlevl]

I guess it looks good to me, but as I said, I'm not the one to decide :)

[Eugeny]

great job @ricott1 and thanks for reviewing @lowlevl !

[Eugeny]

@all-contributors add @ricott1 for code

[allcontributors]

@Eugeny

I've put up a pull request to add @ricott1! 🎉