Changed std::vector<MYSQL_SOCKET> listen_sockets to Dynamic_array

Main reason for this was there was a crash in shutdown of the server
in binlog_encryption.encryption_combo-mix and some other tests because
something in listen_sockets where not initialized. Changing to
Dynamic_array caused things to work.
Other reason for removing std::vector was that it is harder to debug,
and not integrated with DBUG, safemalloc, valgrind or memory
calculation and cause code explosions (extra code generated for each
std::vector type used).

mysys/array.c

@@ -125,6 +125,8 @@ my_bool insert_dynamic(DYNAMIC_ARRAY *array, const void * element)
 void *alloc_dynamic(DYNAMIC_ARRAY *array)
 {
   DBUG_ENTER("alloc_dynamic");
+
+  DBUG_ASSERT(array->size_of_element);          /* Ensure init() is called */
   if (array->elements == array->max_element)
   {
     char *new_ptr;