See if the ledger can be switched to an OptimisticTransactionDB

[evanmcc]

Although we're currently blocked on the RocksDB side (see https://github.com/facebook/rocksdb/issues/2343), if that issue ever gets fixed, it would be nice to see if this works, because it would allow us to drop our current transaction mechanism, which is perhaps not the fastest and quite complex.

[evanmcc]

see also https://github.com/facebook/rocksdb/issues/6814 and maybe https://github.com/facebook/rocksdb/pull/7214

[UrsaarDev]

Although we're currently blocked on the RocksDB side (see facebook/rocksdb#2343), if that issue ever gets fixed, it would be nice to see if this works, because it would allow us to drop our current transaction mechanism, which is perhaps not the fastest and quite complex.

That's possible.