Consider changing integration tests to use the GRPC interface instead of lncli

[aakselrod]

As Joost pointed out (bottlepay/lndsigner#11 (comment)), the GRPC interface to lnd's API is more stable than the CLI interface provided in lncli. However, using it would require either copying ever-larger chunks of the API spec as we expand integration tests, or importing them from lnd wholesale, which we've been trying to avoid. We should make a decision on this going forward.