Add indexer service

[idky137]

This PR ties together work from the previous several PRs implementing FetchService and the mempool update by adding a generic IndexerService and updating zaino-serve to use this struct.

• Add generic IndexerService and IndexerServiceSubscriber structs to zaino-state::indexer.rs.
• Adds IndexerService to zainod::indexer::Indexer.
• Adds IndexerServiceSubscriber to zaino-serve::{Server, Workerpool, Worker, GrpcClient}.
• Updates impl CompactTxStreamerServer for GrpcServer to use IndexerServiceSubscriber.
• Propagates changes throughout code.
• Removes outdated code from zaino-fetch and zaino-serve.

[idky137]

Client_rpc tests in integration-tests are currently non-functional. It has been decided that zaino dev should continue while these tests are being fixed.

[AloeareV]

Reviewed via screenshare!