Convert RPC client to PycURL

Instead of using our custom HTTP client, using PycURL's multi
interface allows us to get rid of the HTTP client threadpool.
The majority of the code is still in the ganeti.http.client
module.

A simple per-thread HTTP client pool gives cURL a chance to
cache and retain as much information as possible (e.g. SSL certs).
Unused HTTP clients (e.g. due to removed nodes) are deleted after
25 requests going through the pool.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>