Investigate CompactionFilter for purging expired entries in RocksDB

RocksDB allows the application to process/delete entries during compaction, and we could use this CompactionFilter callback to implement the purging of expired entries.

It's not straightforward, through: CompactionFilter must be implemented in a native (C++) library. rocksdb-jni allows using a native implementation from Java, but it does not allow defining it in Java (like it does for comparators).

Potential solutions:

1. Write a native library implementing CompactionFilter for our current storage format
2. Change the value format to match the Cassandra storage format and use the predefined CassandraCompactionFilter
3. Revive the now-removed LuaCompactionFilter and write our filter implementation in Lua
4. Contribute an adapter for Java CompactionFilter implementations to the RocksDB project, inspired by the existing AbstractComparator