Username registration messages should be posted to orbitdb, so that user doesn't have to sync blockchain

[holmesworcester]

This is one step in moving username registrations away from Zcash. (Part of #787.)

In this step, we post username registrations (which are signed and will be verified by each user's Zbay) to a username registration log (EventStore?) in Orbitdb.

Acceptance criteria: user registration messages are now retrieved from Orbitdb and not Zcash. (At this point, we only use Zcash for sending and receiving money.)