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.

This change was made because different buckets can have vastly different usage patterns -- even within the same transaction. For example, if one bucket is append-only then it's fill percent should be near 100%. If another bucket is mostly random inserts then the default fill percent (50%) should work well.

/cc @snormore @mkobetic
Added a clarification: cb170160622ce2b68761f58b2dead5a9da1a65c7