This PR adds `--rocksdb-ledger-compression` as a hidden argument to the validator
for specifying the compression algorithm for TransactionStatus. Available compression
algorithms include `lz4`, `snappy`, `zlib`. The default value is `none`.
Experimental results show that with lz4 compression, we can achieve ~37% size-reduction
on the TransactionStatus column family, or ~8% size-reduction of the ledger store size.
ae75b1a25f