RocksDB compaction relative

**QUESTION**
When we use RocksDB as storage for index info, we treat 'ledgerId + entryId' as key in RocksDB. Imagine one case, we have too much ledgers, after put one ledger's index into RocksDB, we put another ledger's index info to it, then RocksDB has to sort these data according to 'ledgerId + entryId' key, these sorting and following compaction will cause some overhead. As @sijie suggested, this [compaction strategy ](https://github.com/facebook/rocksdb/blob/master/include/rocksdb/utilities/table_properties_collectors.h#L23-L27) in RocksDB is helpful, although we haven't active this by default. If I‘m not mistaken, @merlimat introduced RocksDB as a storage for index, so call for your analysis too.