Support for compaction filtering

RocksDB allows us to specify a custom compaction filter that is set when the database is opened. This compaction filter would need to read the record's value and determine if it's expired or if the record is a deletion record and the version is older than the retention time.

```
options.compaction_filter = new CustomCompactionFilter();
// or
options.compaction_filter_factory.reset(new CustomCompactionFilterFactory());
```

Docs: https://github.com/facebook/rocksdb/wiki/Compaction-Filter