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.
This replaces `panic` which aborts aptly execution with warning
message on console. So aptly continues publishing actions, but
`Contents` indexes might be incomplete.
Error will be printed every time contents generation is triggered.
