first attempt at read my writes (more or less necessary to have with a hashtable...)
throw Abort() + make add_item<false> easier to use
remove debug print statements
fix a few bugs. seems to be working fine now, though time of completion can be highly variable (possibly due to aborts??)
modified concurrent.cc to support Hashtable testing (uses integer keys/values). should also write separate hashtable specific tests for more complex key/value types (e.g. strings)