[chain] Make KeychainTxOutIndex more range based

[LLFourn]

KeychainTxOut index should try and avoid "all" kind of queries. There may be subranges of interest. If the user wants "all" they can just query "..".

The ideas is that KeychainTxOutIndex should be designed to be able to incorporate many unrelated keychains that can be managed in the same index. We should be able to see the "net_value" of a transaction to a specific subrange. e.g. imagine a collaborative custody service that manages all their user descriptors inside the same KeychainTxOutIndex. One user in their service may pay another so when you are analyzing how much a transaction is spending for a particular user you need to do analyze a particular sub-range.

Notes to the reviewers

• I didn't change unused_spks to follow this rule because I want to delete that method some time in the future. unused_spks is being used in the examples for syncing but it shouldn't be (the discussion as to why will probably surface in #1194).
• I haven't applied this reasoning to the methods that return BTreeMaps e.g. all_unbounded_spk_iters. It probably should be but I haven't made up my mind yet.

This probably belongs after #1194

Changelog notice

• KeychainTxOutIndex methods modified to take ranges of keychains instead.

Checklists

All Submissions:

• [x] I've signed all my commits
• [x] I followed the contribution guidelines
• [x] I ran cargo fmt and cargo clippy before committing

New Features:

• [x] I've added tests for the new feature
• [x] I've added docs for the new feature

[ValuedMammal]

If I understand, you're using range in a couple different ways: in keychain/txout_index to range over keychains, and in spk_txout_index to range over derivation indexes - with map_to_inner_bounds having some mediating role

Am I right in thinking these are similar but distinct concepts? or is it just two ways of looking at the same thing?

[LLFourn]

If I understand, you're using range in a couple different ways: in keychain/txout_index to range over keychains, and in spk_txout_index to range over derivation indexes - with map_to_inner_bounds having some mediating role

Am I right in thinking these are similar but distinct concepts? or is it just two ways of looking at the same thing?

SpkTxOutIndex they are similar. KeychainTxOutIndex is an SpkTxOutIndex with a specialized structure where each group of spks is under something called a "keychain". In SpkTxOutIndex each spk is under its own unique key. In order to query across multiple keychains in KeychainTxOutIndex, you need to be able to query across sub-ranges of indices in SpkTxOutIndex.

[notmandatory]

Do we have a current need for this or can it be moved to a post 1.0 release as a non-breaking internal optimization?

[LLFourn]

It's an API change for bdk_chain and doesn't effect bdk_wallet I don't think. it doesn't need a milestone since if I understand that system is only meant for prioritizing tasks with respect to bdk_wallet where as the order of other tasks is left to the discretion of the devs.