[http-server-javascript] Rework serialization logic to be more HTTP-specific.

When I first designed the serialization layer for the HTTP server generator for JS, I designed it to be protocol-agnostic. However, this doesn't really work well for more complex HTTP envelopes, and it became clear to me that HTTP-aware serialization code is required. This issue tracks reworking the existing (de)serialization logic to handle complex HTTP envelopes well.