RocksDB clear always reinitializes the database

The RocksDBStore always destroys the database on a clear and reinitializes it. This causes a lot of disk IO, especially when the store may have very few entries. The configured clearThreshold is only supposed to reinit the database if so many entries are found, but the variable should have been checked as > instead of <=.

if (optionalIterator.isPresent() && configuration.clearThreshold() <= 0) {