zcash_client_backend: Introduce "chain access API" to the sync engine

[str4d]

Currently the sync engine (imported from zec-sqlite-cli) is tightly coupled to lightwalletd. Once we have determined what additional kinds of information the sync engine will need to consume in order to replicate the zcashd wallet (#1395), we should introduce an interface trait for obtaining that information (with the initial implementation being the current lightwalletd gRPC client code, where anything we can't get from lightwalletd either returns a "not available" sentinel, or is contained in a separate extension trait).

[str4d]

This will be easier to do once we can mock out the wallet backend (#1415).