User should be able to register account without Zcash blockchain

[holmesworcester]

Right now, our system for registering accounts depends on Zcash. This has downsides:

1. Depends on central servers (zecwallet's lightwalletd, and our faucet/registration node).
2. Trusts a central server, to some degree.
3. Has different privacy and anonymity properties than Tor, and leaks IP address in relation to username.
4. Inevitable delays in registered usernames becoming visible.
5. Requires running a zcash node in the cloud and funds

We want a way to register accounts that does not depend on Zcash.

These are the current proposals: #721 (comment)

For this issue, the simplification is that we don't have to introduce a notion of multiple accounts or networks. We just need to switch from the existing username registration scheme based on Zcash to a new one based on waggle, with our cloud waggle node as the "owner" and the source of truth.

I think the first step is to do everything as we currently do, except build the piece of the cloud node that signs username registrations, and adds them to an orbitdb of usernames and keys.

UPDATE: I've broken this up into multiple tickets which can each be released independently:

1. All user registration messages posted to blockchain should be signed by our node #813
2. Zbay should check that every username registration message is signed by our node #814
3. Username registration messages should be posted to orbitdb, so that user doesn't have to sync blockchain #815
4. An always-on Zbay (not our cloud node) should handle username registrations #816