Use v3 hidden services (alongside v2 if a service already exists)

This branch implements the next stage of the migration from v2 to v3 hidden services.

If the account already has a v2 hidden service, v2 and v3 services are run in tandem for 6 months to allow contacts to receive the v3 address, after which the v2 service is retired. If the account doesn't already have a v2 hidden service, only a v3 service is created.

The code for connecting to v3 addresses was released in October 2018, so most users should have upgraded by now.