ganeti.http: Split HTTP server and client into separate files

This includes a large rewrite of the HTTP server code. The handling of
OpenSSL errors had some problems that were hard to fix with its
structure. When preparing all of this, I realized that actually HTTP
is a message protocol and that the same code can be used on both the
server and client side to parse requests/responses, with only a few
differences. There are still a few TODOs in the code, but none should
be a show stopper. Many pylint warnings have been fixed, too.

The old code will be removed once all users have been migrated.

Reviewed-by: amishchenko