try to tweak async client API
Rename: `keyify?` -> `keywordize-headers`, `transform-header` -> `normalize-headers`
Remove deprecated stuff: `gen-handler`, `get`, `post`, Text/Binary class imports
Clean up client initialization stuff. Move :timeout and :user-agent opts from clients to requests.
Add low-level async `request*` fn
Modify `request` macro to use new low-level async `request*` fn
Modify `request*` to support both promise and callback mechanisms
Modify `request` macro to use new low-level `request*` callback mechanism
Remove commented stuff