3.2 Implement lightwallet service using ReadStateService instead of the JSONRPC interface

[zancas]

Re-implement the lightwallet gRPC service using Zaino-State (backed by Zebra's ReadStateService). Legacy functionality relying on Zaino-Fetch (backed by Zebra's JsonRPC service) should be kept as a configurable option.

• Blocked on 2.1 - Implement Zcash RPC functionality required by lightwallet services, backed by Zebra's ReadStateService #54, 2.3 - Provide efficient Compact Block fetch and access to a local non-finalised state for backup RPC backend. #55, 2.4 - Provide interface to / provide internal representation of mempool from Zebrad #56

Tasks

• Task 3.2.1: Implement lightwallet gRPC services using Indexer trait defined and implemented in Milestone 2.
• Task 3.2.2: Make fetching service configurable (StateService or FetchService) on system launch.