Index Mempool

[lifofifoX]

This is perhaps the final missing piece before ord can fully replace electrs for most use cases. Certainly, all of mine.

At a minimum, we need:

• --index-mempool flag as an opt-in
• Index all pending outspends
• Include pending outspends wherever outputs/UTXOs appear in API responses
• Index all mempool TXIDs
• Index mempool TXIDs by address

Additionally, the following would be nice to have:

• Track rune movements in the mempool
• Track sat movements in the mempool
• Make ^^ tracking work for chain of pending TX

Re: implementation, I think indexing can poll getrawmempool every couple of seconds and sync the mempool index. And perhaps clear it all and rebuild when it detects a height change.

This might be a bit too complicated with my current Rust experience, but I'm happy to slowly work on this if there's any appetite to have it in ord. But we're probably talking months here 😅