MXS-2197: Make SERV_LISTENER a C++ class

The class is still mostly the same as the old C version but it now uses
std::string instead of char pointers. Changed configuration default values
so that the parameters passed to the listener allocation are always valid.

server/modules/authenticator/CDCPlainAuth/cdc_plain_auth.cc

@@ -556,8 +556,6 @@ int cdc_replace_users(SERV_LISTENER* listener)
         int i = cdc_read_users(newusers, path);
         USERS* oldusers = NULL;
 
-        pthread_mutex_lock(&listener->lock);
-
         if (i > 0)
         {
             /** Successfully loaded at least one user */
@@ -578,8 +576,6 @@ int cdc_replace_users(SERV_LISTENER* listener)
 
         cdc_set_service_user(listener);
 
-        pthread_mutex_unlock(&listener->lock);
-
         if (oldusers)
         {
             users_free(oldusers);