datamap: Split sync plan from execution
datamap: Split sync plan from execution

To reduce memory usage, split the sync plan from executing on the sync
plan, reset the S3 client, and stop tracking the total files that do
not change rather than the filenames.

This also rearranges code to match the function declaration pattern,
which makes for a busy diff.

mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea,mozilla/ichnaea