Deferred delete bug (aka Cd8)

This is work leftover from #156.

* Commit 948fc84 has a non-concurrent EUnit test case for the problem
* Commit 944cef8 has a fix that Engel despises.  :-)

Let the kibbitz commence.

This is the one think I would like remove from this patch.  I would rather just store the biggest file id as is, without adding some magic number to it.  This kind of fudge just helps hide issues for longer than they should and adds strange behavior that would be hard to account for in test validations, etc.