Support for DocIdSetBuilder with (min,max) docId

### Description

Currently, we are using `maxDoc >>> 7` as the default threshold before upgrading to BitSet. I am wondering if there is more optimal way of determining the right threshold.

The memory cost of `FixedBitSet` is always known, and we can get the selectivity from `(# docs in buffer)/(maxDoc in buffer)` so far. If the selectivity is high, consider upgrading to `FixedBitSet` earlier, else should delay the upgrade.