Create a new DFP session before each run

Hopefully, this will allow all the previous objects to get flushed from
memory
Merge branch 'master' into optimisingDfpFetcher
DfpDataHydrator is now a class...

Which you new up just before you do a large DFP job.
Removed debugging line
Merge pull request #6833 from guardian/optimisingDfpFetcher

Create new DfpSession for each scheduled job