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.
See #765, #761
Collections were relying on keeping in-memory list of all the objects
for any kind of operation which doesn't scale well the number of
objects in the database.
With this rewrite, objects are loaded only on demand which might
be pessimization in some edge cases but should improve performance
and memory footprint signifcantly.
