Receive and send big files

[gwolf-011235]

Right now we store the full request (header and body) and full response (header and body) in an std::string.

With big files (e.g. 2GB) this is a problem:

1. if we want to receive a big file the buffer won't hold the full thing
2. likewise if we want to send a big file - buffer won't hold it.

Possible solutions:

1. save the body as a temp file.
2. don't cache the body of the response in the buffer. Just open it and send directly.