The execution of OperationContext::checkForInterrupt() is currently independent of whether true or false is passed for the "heedMutex" argument ("heedMutex" is only observed if Client::hasWrittenSinceCheckPoint() is true, but it is never true since Client::writeHappened() is no longer called). The "heedMutex" argument should be removed.

It used to be the case that "heedMutex" controlled whether or not an operation was interruptible after its first write; since writes could not be rolled back, it was important to forbid interrupting partially-completed writes. Now that writes always happen in rollback-enabled containers (WriteUnitOfWork), it should always be safe to interrupt writes.