Replace `cancelled` property with `isCancelled` method
Replace `cancelled` property with `isCancelled` method

`cancelled` property is only available on targets iOS 8+ and OS X 10.10+
replacing with its getter `isCancelled`

Signed-off-by: Jimi Xenidis <76b363d276dba42912c2eb4a024728f40d1d8033@pobox.com>

phatblat/CDTDatastore,phatblat/CDTDatastore,cloudant/CDTDatastore,cloudant/CDTDatastore,phatblat/CDTDatastore,jhs/CDTDatastore,jhs/CDTDatastore,jhs/CDTDatastore,cloudant/CDTDatastore,cloudant/CDTDatastore