BrooklynNode entity safe STOP effectors

Effectors to stop BrooklynNode forcing the user to choose explicitly whether to stop the running apps or not. Refuse to STOP if there are existing children (possibly mirroring nodes added by another entity)
Duration.parse returns null on null input. Null value for httpReturnTimeout results in no request blocking, to preserve exiting behaviour (but the behaviour changes in #172).
Empty string will result in parsing error which I think is a good thing. Better to keep the behaviour identical to TypeCoercions logic. Though a Preconditions check would be better in this case, I will add the checks to #172.