Request transaction abort method

The Transaction interface should have a method that any thread can use to request that the given transaction be aborted (similar in spirit to interrupting a thread). This would either fail if the transaction has already started committing, or succeed and mark the transaction such that the next attempt to access its state would result in an exception being raised.

------- Additional comments from stp Fri Feb 20 17:41:05 +0000 2009 -------

After discussion with Tim we decided that as a simpler starting point this functionality will be implemented as an internal utility of the Access Coordinator. This means that we don't have a fully cooperative system that could make aborts happen more efficiently, but constraining the implementation will make it easier to support and give us a better sense of how this feature might be generalized. The ticket is being left open because it's quite likely that in the future we will want to expose this feature more broadly.