RFC on mt_allocator.h

[Benjamin Kosnik]

>The other test (allocator_thread.cc) does a good job of exercising 
>multithreaded heap access, but each of its container elements is 
>accessed exclusively by a single thread.  That's not very realistic, 
>but I can't quite make the case for adding a producer-consumer test, 
>even though it would have been my first choice.

We could add a new producer-consumer test. Do you have one in mind?

-benjamin