Change fill percent to be per-bucket.

This commit moves the DB.FillPercent field to Bucket.FillPercent. This
allows the fill percentage to be specified per-bucket, per-tx. This
value is not persisted and should be set whenever using it.
Add FillPercent documentation.
Merge pull request #229 from benbjohnson/bucket-fill-percent

Change fill percent to be per-bucket.