sasl_server_step() performs a step in the authentication negotiation. It returns SASL_OK
if the whole negotiation is successful and SASL_CONTINUE if this step
is ok but at least one more step is needed.
conn is the SASL connection context
clientin is the data given by the client (decoded if the protocol encodes requests sent over the wire)
clientinlen is the length of
serveroutlen are set by the library and should be sent to the client.