Return a more useful recommended poll time from getNextFileURL method
The new getNextFileURL interface method returns a suggested poll interval. However, it's just returning a fixed value. We ought to return some value that depends on the likelihood that this project will get another file in the near future (so longer intervals if everything is coming from tape), and the system load.
(Currently SAMWeb doesn't make any use of this value anyway, due to deficiencies in the CORBA client code. If this gets done before we implement http directly in the station, the SAMWeb server will need to be updated too.)