Improve feature metrics join

• fix bug in the feature metrics logic that ensures there is a row for each wiki/page/time_bucket, e.g. in case no feature metrics exist for a given month.
• improve the get_all_timebuckets logic to only create time_bucket entries for buckets after the article creation time_bucket. This had a much larger impact than I thought on file size and runtime.
• add test for get_all_timebuckets