Refactor SASL exchange to return tri-state status

Refactor SASL exchange to return tri-state status

The SASL exchange callback returned state in to output variables:
done and success.  This refactors that logic by introducing a new
return variable of type SASLStatus which makes the code easier to
read and understand, and prepares for future SASL exchanges which
operate asynchronously.

This was extracted from a larger patchset to introduce OAuthBearer
authentication and authorization.

Author: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://postgr.es/m/d1b467a78e0e36ed85a09adf979d04cf124a9d4b.camel@vmware.com