When aptly crashes it is possible to get a corrupt database with a dangling key reference.
This results in an error with 'key not found', eg:
ERROR: unable to load package Pall example-package 1.2.3 778cf6f877bf6e2d: key not found
This change makes `db recover` fix this situation by removing the dangling references.
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.
