calculate the content lenght of a multipart stream.

Some servers like NGINX don't handle a multipart stream sent with a
chunked encoding request. This change add the possibility to calculate
the full content length before sending the parts.

breaking changes:

- hackney_multipart:encode_form/1 has been rewritten to take new messages so we can return the content-length with the binary.
- mp_header/4 has been removed