Move builders to be move by value

[LucioFranco]

The http crate with its 0.2 release will move its builders to be from &mut self to self. I want to consider following suit with tonic. Mostly, I wanted to open this to start a discussion.

cc @jen20

[jen20]

Personally I'd prefer them to use self over &mut self - that should remove the need to working out where clone is needed.