Stop clobbering detailed BodyMedia data

After running the `*_day` synclets, the `*_year` synclets would rewrite the
dates on their next run with less detailed data. Because the `_day` version
had marked the day done, it would never get recrawled. This prevents the
`_year` version from returning data for days that have been crawled by
`_day`.

As a side note, these synclets are nearly identical and should be refactored.
Because we crawl forward in time, we could probably just store the latest
date too, rather than the whole set of finished days.

@jparkrr, care to review?