Historically, controldb reads could be treated as just a datastructure,
but that became a lie when reconnections were introduced.

Some tricks were employed to enter the async context when needed, but
those always bear the risk of introducing a deadlock somewhere.

So, just make it async.

Expected complexity level and risk

2

It may or may not be safe to use sled in an async context. We did already for
the write path, but if it's a problem it'll show now.

Testing

Not a functional change.