Removed ValuePlug::Computation::resultWritten.

It dates back to a time when the results of computations were allowed to be NULL - since they are now guaranteed to be non-NULL we can just test m_resultValue to see if it has been written.
Fixed threading issue in ValuePlug caching code.

We were calling cached() and then calling get(), but in the moments in between, another thread could have removed the item we wanted from the cache. The new test cases would either throw exceptions from the nullGetter() or they would hang - now they don't.
Merge pull request #689 from johnhaddon/valuePlug

Cache threading fix