[Java Client] can erroneously log "Unable to load custom SASL providers." when loaded with multiple class loaders

An ERROR log "Unable to load custom SASL providers." is produced by the java client when CallbackHandlerRegistry is loaded by multiple class loaders. In this case the first class loader successfully registers the SASL providers on opening of the first connection and uses of other class loaders causes the error message because they are enable to register SASL providers because they are already registered.

The registration process should be updated to detect that all required providers are already registered and only log an error if they are not.