Make Mappings export more atomic

At the moment, if you happen to download the file during the 42 minute window
in which it is being generated, you will get a partial file.

With this change, whilst the same user might get an older file, they will at
least get a complete one.
Merge pull request #1233 from alphagov/make_mappings_export_more_atomic

Make Mappings export (more) atomic