Rename `Indexer` to `TxOutIndexer`.

[evanlinjin]

Describe the enhancement

I think this is a more descriptive name. Additionally, I think if we plan on indexing any other part of the transaction, it should be a separate trait (or a separate thing).

[LLFourn]

I don't think TxOutIndexer a more descriptive name of a trait which has a method called index_tx. If I were to rename it to anything is would be TxIndexer sine the method index_txout is rather secondary -- it's only used for indexing dangling txouts when you don't have the full tx and its not even clear if we should be indexing those or just using them to determine fees.