MXS-862: Move sending of OK packet to mysql_client

Moving the sending of the final OK packet of the authentication process to
the client protocol plugin makes the authentication plugins simpler.

By reading the client's sequence and incrementing that by one, the client
protocol module will always send the correct sequence byte in the final OK
packet.

server/modules/authenticator/gssapi_auth.c

@@ -281,8 +281,7 @@ int gssapi_auth_authenticate(DCB *dcb)
 
         MYSQL_session *ses = (MYSQL_session*)dcb->data;
 
-        if (validate_gssapi_token(ses->auth_token, ses->auth_token_len) &&
-            mxs_mysql_send_ok(dcb, 4, 0, NULL))
+        if (validate_gssapi_token(ses->auth_token, ses->auth_token_len))
         {
             rval = MXS_AUTH_SUCCEEDED;
         }