atomtable: more dbg statements
unit test: Use the official atomspace API.
atomsapce test: extend the threading test.
Atomspace: add an extended deletion test.
test: add a note ...
Try to make the unit test more challenging.
try to make test case even harder.
Another test attempting to elicit bad behaviour.
Atomspace: Loosen atom removal error check.

I cannot get the unit test to trip over this, but clearly, Shujing's
code does, in the trace in bug report #1025  I can see the root
cause of the race, and what's weird is that the unit test never
trips on it.  That's the part I cannot figure out ...
Merge pull request #1033 from linas/dbg

Extended unit test for bug #1025