KManageSieve::Session: move network communication to a thread

This moves all code in KManageSieve::Session that deals with KTcpSocket and
SASL to a separate thread. The reason for this is that on a very bad/slow
internet connection the initial SASL handshake blocks in sasl_client_new(),
apparently due to synchronous DNS resolution. Handling the communication in
the main thread then freezes KMail on start as the Sieve code tries to check
for the Vacation script. Moving the SASL communication to a thread solves
the problem.

REVIEW: 125748