http.client: Disable SSL session ID cache

This patch disables the SSL session ID cache for all cURL operations.
This is needed because http.HttpBase's PyOpenSSL implementation does not
currently set a context using SSL_set_session_id_context(3SSL), cURL
tries to re-use the session ID and, according to
SSL_set_session_id_context(3SSL):

 If the session id context is not set on an SSL/TLS server and client
 certificates are used, stored sessions will not be reused but a fatal
 error will be flagged and the handshake will fail.

Ideally, session caching should be either controlled, or disabled in
HttpBase, however PyOpenSSL does not seem to implement
SSL_CTX_set_session_cache_mode nor SSL_CTX_set_session_id_context which
are used for these purposes (it seems that only M2Crypto's SSL module
supports these).

Signed-off-by: Apollon Oikonomopoulos <apollon@noc.grnet.gr>
Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>