[racket] Getting unusual behavior from racket/sandbox: not truly capping memory usage?!

Ok, good!  Matthew Flatt is investigating.

It looks like something like:

    https://github.com/dyoo/racket/commit/6547d18c006e7390c3cbce84a93dd9d9c926341b

can clear the memory leak, but mflatt says that we shouldn't have to
do this, that the VM should automatically kill the monitoring thread
once it detects the nack is inaccessible.  This is totally out of my
domain, but thankfully he's on it now.  :)