This changeset performs some internal refactoring and cleanup. In particular, it removes the duplicate Client\Response class that has been introduced temporarily via #368.

Besides improving our internal code structure, this is also useful because IDE autocompletion no longer suggests this class when dependents are looking for a response class. The correct class continues to exist in Message\Response as of #370. Accordingly, this does not affect the public API, so this is not a BC break.

Additionally, this is also needed in preparation for HTTP keepalive support (#39) and client-side connection upgrades (#376). A longer-term goal is to remove the entire Client namespace (entirely internal).

Builds on top of #372, #380, #381 and others.