Internal refactoring and improvement to hold message body in memory

[clue]

This changeset only performs some internal refactoring and minor improvements to reuse the existing BufferedBody class (introduced via #395) to hold any message body in memory.

Prior to this change, it leaves this up to the underlying PSR-7 implementation which happens to use a temporary memory stream. With these changes, we no longer need to interface with streams and resources internally which happens to improve performance slightly (9200 req/s -> 9300 req/s). On top of this, dumping an HTTP message now shows the actual body contents instead of a resource, so this definitely makes debugging much easier.

This also makes us less dependent on the underlying PSR-7 implementation which allows us to possibly change this in the future (#331).

These changes do not otherwise affect the public API.

Builds on top of #395