Tracking: Enable `zebra-scan` to run as a separate independent process from `zebrad`

[mpguerra]

Motivation

In order to keep private key material outside of zebra, we need to decouple zebra-scan functionality from zebrad.

Design

Some diagrams for the design can be found in https://github.com/oxarbitrage/zebra-grpc-scan-spec/blob/main/zebra-scanner-process.md

We want to get started with the finalized state (reading from the database) part of the blockchain:

• [x] The scanner will communicate with Zebra for the finalized part of the blockchain with a ReadStateService::init method: https://github.com/ZcashFoundation/zebra/issues/8520
• [ ] We need a replacement for ChainTipChange so we can use it in zebra_scanner::spawn_init: https://github.com/ZcashFoundation/zebra/issues/8580
• [ ] Create a new binary for the new process: https://github.com/ZcashFoundation/zebra/issues/8581
• [ ] Get rid of scanner startup code from zebrad: https://github.com/ZcashFoundation/zebra/issues/8582
• [ ] Move or remove the possibility of keys in the scanner: https://github.com/ZcashFoundation/zebra/issues/8584
• [ ] Testing: https://github.com/ZcashFoundation/zebra/issues/8585
• [ ] Documentation: https://github.com/ZcashFoundation/zebra/issues/8586

Non finalized state:

• [ ] https://github.com/ZcashFoundation/zebra/issues/8576
• [ ] https://github.com/ZcashFoundation/zebra/issues/8633

We should also evaluate if it makes sense to keep the zebra-scan crate inside the zebra repo as it is now or whether we need to move it to another repo.

Related work

This a step towards enabling zcashd deprecation by replacing the zcashd built in wallet with a zebra backed equivalent as per zcash/librustzcash#1373 .

[oxarbitrage]

Related:

• https://github.com/ZcashFoundation/zebra/issues/8141
• https://github.com/ZcashFoundation/zebra/issues/7937
• https://github.com/ZcashFoundation/zebra/issues/7926

[mpguerra]

We've finished all the required tasks here