Fixed race condition in Disposable.Dispose()

Disposable.Dispose() and Owned<T>.Dispose(bool) were using
Interlocked.CompareExchange but its return value was not used. The
disposal was driven by a cached local variable instead of cmpxchg result
- the actual synchronized value of the field.
For simplicity I've changed CompareExchange to Exchange only.
Merge pull request #545 from pifil/master

Fixed race condition in Disposable.Dispose()