implement basic X-Forwarded-For header support

The commit introduces two major things:
1. The HTTP parser is now able to parse the X-Forwarded-For header and
   put it to the headers table, although t doesn't parse IP addresses
   or distinct Node IDs yet.
2. Client's IP address is added to X-Forwarded-For header of each
   forwarded HTTP request. If there is no such header - it is created,
   if it is already there, then the client's IP is appended to the list.

Also the commit contains fixes for the HTTP parser that used to crop
parsed HTTP headers and write them into each other's locations in the
headers table.

Related to: #5 - Load Balancing