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Switch to using a more generic credentials_callback/handler for SASL auth. Not all authentication mechanisms have the same requirements; it makes sense to provide them only with the information they require (and for them to depend on that) so that as many auth mechanisms as possible can be supported with a variety of credentials-storing schemes. This commit patches that together

== Core development team ==

Matthew Wild (matthew.wild AT heavy-horse.co.uk)
Waqas Hussain (waqas20 AT gmail.com)