Replace `BatchRunner` with `zcash_client_backend` implementation

[str4d]

In #6088 we landed a BatchRunner that implements multithreaded batched trial decryption of full outputs. This was subsequently duplicated and tweaked in the zcash_client_backend crate for doing the same with compact outputs. Once zcash/librustzcash#1171 is complete, we can de-duplicate this code again by adding a dependency here on zcash_client_backend.