Cancelling a `RecursiveDirectoryMonitor` observer doesn't guarantee it never receives callbacks

[jbmorley]

While it shouldn't cause any bugs at the moment, we should consider making it a hard-and-fast guarantee that observers never receive callbacks once their cancel method returns.

[jbmorley]

I've now added isCancelled property to the context which can be used to track whenever an observer is cancelled to ensure that no further calls are made to the observer blocks.