Revert "Change SASL handler computation type"

This reverts commit aadd0335978999734c6591772c88887b2611edff.

Commit "Change SASL handler computation type" introduced a deadlock
when sasl handlers tried to take the TMVar that was already taken by xmppSasl

The SaslM computation type _has_ to be a State transformer rather than working on the TMVar directly because otherwise we would either have to acquire the same lock twice (resulting in a deadlock; situation before this patch) or release the lock prematurly, which would allow the authentication to be preempted (introducing a race condition)

Conflicts:
	source/Network/Xmpp/Sasl.hs
replace isJust / fromJust with pattern matching
replace "debug" constant with hsLogger functionality in Network.Xmpp.Tls
Adapt EchoClient to changed API