* Optimize for leap array

- Fix a bug: old position is not cleaned when inserting into a new (empty) position
- Reuse buckets for optimization
- The strategy is now changed: deprecated buckets will not be reset until newer time triggered. LeapArray is responsible for filtering the deprecated buckets (e.g. in `list` or `values`)
- Update test cases

Signed-off-by: Eric Zhao <sczyh16@gmail.com>