Major Description
This change adds support for soft deletes as a fully supported feature by the index writer. Soft deletes are accounted for inside the index writer and therefor also by merge policies.
This change also adds a SoftDeletesRetentionMergePolicy that allows users to selectively carry over soft_deleted document across merges for retention policies. The merge policy selects documents that should be kept around in the merged segment based on a user provided query.