refactor accounts system so that buntdb is no longer the data plane

[slingamn]

This relates to #1971. In almost every place we use buntdb, it is not the data plane: it is not queried at runtime because the relevant data exists in native Go datastructures. This has the advantage that runtime latencies are not sensitive to the latency of the database layer (so we could use a much slower database layer, without impacting performance other than startup time).

The one exception is accounts:

• The source of truth about whether an account name is registered is buntdb
• Account login always retrieves credentials and settings from the database

Accounts should be refactored so they're like other systems (channel registrations, [DK]-lines, etc): the source of truth should be on the Go heap behind a sync.RWMutex, not in the database.

[KellerFuchs]

Wouldn't that make #26 (s2s) harder, as the link protocol would need to do (something like) distributed cache coherency, to ensure there's no inconsistency between in-heap data and the DB?

[slingamn]

See #1532 for current work on this.