track and limit memory usage per connection

We want to be able to know how much memory a particular connection was allocated. And Limit this amount, if necessary.

Possible implementations (all in my_malloc()/my_free()):

• allocate 16 bytes more and store the size there
• use _msize (windows), malloc_usable_size (linux), malloc_size (mac os x). That's simpler, but is it portable enough for us? See also this thread