Support Chunked Messages in HTTP Header Filter

[t4lz]

Currently we collect the whole message (request in the agent, response in the layer) before forwarding it, which means we do not support real streaming over HTTP with the HTTP Header Filter.

We should introduce new messages to the mirrord protocol, and if a message is chunked, forward the chunks in both directions as they come.

[aviramha]

Need to understand if this is relevant to HTTP2 as well - in that case, have HTTP2 as another issue.

[gememma]

The diferences between HTTP1 and 2 are irrelevant at the point where this gets handled, so no special considerations are needed. I will however create separate issues regarding HTTP requests and HTTP responses.