2.4 - Provide interface to / provide internal representation of mempool from Zebrad

[idky137]

Due to requirements of the get_mempool_stream gRPC service it is more efficient to keep an internal mempool rather than serve mempool data directly. Depending on functionality the IndexerMempool will either use the ReadStateService or Zaino-Fetch as its backend. If possible it should be configurable to use either.

• Task 2.4.3 Blocked on 2.1 - Implement Zcash RPC functionality required by lightwallet services, backed by Zebra's ReadStateService #54
• Task 2.4.4 Blocked on 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 #122
• Task 2.4.5 Blocked by Task 2.1.3: Add Indexer Trait #112
• Task 2.4.6 Blocked on Upgrade Zaino-testutils to use zcash-local-net. #96

Tasks

• Task 2.4.1: Implement efficient Broadcast stream for high concurrency use case.
• Task 2.4.2: Add Mempool trait.
• Task 2.4.3: Implement Mempool for StateService.
• Task 2.4.4: Implement Mempool for FetchService.
• Task 2.4.5: Add Mempool trait to Indexer trait.
• Task 2.4.6: Add test set that comprehensively tests mempool functionality.