2.3 - Provide efficient Compact Block fetch and access to a local non-finalised state for backup RPC backend.

[idky137]

Zaino currently fetches full blocks and parses them to compact blocks every time a client requests block data. This is very inefficient and access to a local compact block cache or at the very lease an internal non-finalised state is required for scale use. This will be inherited from Zebra when Zaino is backed by the ReadStateService (StateService) but this will still not be the case when Zaino uses its backup RPC backend (FetchService), with zcashd or other future RPC based validators. For this reason we need to implement an internal non-finalised state for FetchService.

• Task 2.3.1 partially Blocked on Task 2.4.1: Implement efficient Broadcast stream for high concurrency use case #116
• Task 2.3.4 Blocked on Upgrade Zaino-testutils to use zcash-local-net. #96

Tasks

• Task 2.3.1: Create FetchService struct in Zaino-State, initially containing Zaino-Fetch's JsonRpcConnector and a non_finalised_state struct, and implement sync method as a persistent background process.
• Task 2.3.2: Implement Indexer for FetchService.
• Task 2.3.3: Add get_compact_block method to Indexer trait and implement for StateService and FetchService.
• Task 2.3.4: Add RPC test for get_compact_block and add RPC tests for FetchService

NOTE: If compact blocks are not available in the ReadStateService at the time this work is being implemented, StateService will continue parse data internally until the required indexes are available.