Fix stickage when disconnecting from broken SSL IRC connection.

Previously, Quassel was calling socket.disconnectFromHost()
when the disconnect timeout expired.  For some reason, when
connected using SSL and if the connection was still open but
no longer sending or receiving data, this function would
become stuck for quite some time before actually
disconnecting.  This patch should fix it by making it use
socket.abort() instead, which will terminate the connection
without any waiting.