OPENDJ-2937 Fix SDK SASL bind requests implementations

* For multi-stage (CRAM-MD5, DIGEST-MD5, GSSAPI and EXTERNAL) mechanisms
    * Rely on the result of saslClient.evaluateChallenge(...) instead of saslClient.isComplete() to know if there is a need to send additinal data to the server (i.e if we are in a multi-stage SASL bind)
    * Factorize code in parent class to prevent duplication.

* As SASL Plain is not  multi-stage we do not need to override evaluateResult() in the bind request implementations, return true is enought.

* Also adapts test ldap server to not systematically attach a challenge to a bind result.