Include all 4 sync/async client/server combos

This extends the existing http parser to support requests as well as responses.

httpcli continues to exist and work as it has previously, though in the new
directory src/core/http (to reflect the fact the directory now contains code
relevant to parsing requests, which httpcli would not generally involve itself
in).

payload config in our server config. This affect any generic server
tests that use anything other than 0-byte responses: essentially,
server-streaming or bidi throughput tests.

Implemented compression level algorithms properly (as a function of the accepted algorithms by the call's peer.