Removed the ability to get a Txn from a NestingLevel.  If necessary we can re-add it, but without it the user can't get into trouble as easily.
Removed the ability to get a Txn from a NestingLevel.  If necessary we can re-add it, but without it the user can't get into trouble as easily.

nbronson/scala-stm,nbronson/scala-stm