This is spin-off of changes from #459.
Transactions are not being used yet, but batches are updated to work
with the new API.
`database/` package was refactored to split abstract interfaces and
implementation via goleveldb. This should make it easier to implement
new database types.
Allow database to be initialized without opening, unify all the
open paths to retry on failure.
In API router make sure open requests are matched with acks in explicit
way.
This also enables re-open attempts in all the aptly commands, so it
should make running aptly CLI much easier now hopefully.
Fix up system tests for oldoldstable ;)
