LoadFirstBlock loads the first indexed block, returns -1 if db is empty.

LoadLastBlock returns the latest indexed block number, returns -1 if db is empty.

NewKVIndexer creates the KVIndexer.

TxHashKey returns the key for db entry: `tx hash -> tx result struct`.

TxIndexKey returns the key for db entry: `(block number, tx index) -> tx hash`.

TxIndexKeyLength is the length of tx-index key.

KVIndexer implements a eth tx indexer on a KV db.