The leaks are all 40 bytes and happens in this call stack when running
mtr vcol.vcol_syntax:

alloc_root()
...
Virtual_column_info::fix_and_check_exp()
...
Delayed_insert::get_local_table()

The problem was that one copied a MEM_ROOT from THD to a TABLE without
taking into account that new blocks would be allocated through the
TABLE memroot (and would thus be leaked).
In general, one should NEVER copy MEM_ROOT from one object to another
without clearing the copied memroot!

Fixed by, at end of get_local_table(), copy all new allocated objects
to client_thd->mem_root.

Other things:
- Removed references to MEM_ROOT::total_alloc that was wrongly left
  after a previous commit