Refactor RocksDB clearThreshold

Currently the RocksDB store utilises a "clearThreshold" to try to delete entries individually before deleting and re-initiating the database. We should deprecate this threshold and always delete/reinit the database.

Currently when deleting the database, we utilise Util.recursiveFileRemove which does not confirm that the file has actually been deleted. Instead, we should provide a nio based implementation instead, similar to the one stated here. This has the advantage that an IOException is thrown by java.nio.file.Files::delete