Test that zcashd and Zebra can sync from Zebra

[teor2345]

Once we've merged GetBlocks and GetHeaders support #1347 , Zebra should be able to serve blocks to other nodes.

We should test that zcashd and Zebra can sync from genesis, using a fully synced Zebra.

This is a network integration test, so its out of scope for the first alpha. But it would be nice to make sure there isn't any really bad behaviour.

[mpguerra]

@ZcashFoundation/zebra-team has anyone tried this already?

[oxarbitrage]

I never tried this but i think is a good idea to sync zcash and zebra only with zebra seeders. We should have this done for the first beta so we should prioritize.

[teor2345]

I run tests like this all the time, and I see Zebra syncing from Zebra.

But I don't do it in an isolated environment where there are no extra zcashd instances.

[teor2345]

This would be a useful test, but it's partly covered by our other tests.