fetcher: provide api to fetch objects without validating if they are available in db

[dshulyak]

existing api always tries to lookup if object exists in the database, this is both inefficient and not robust.

as an example ballot validation

in fact all this objects are available in memory in the tortoise, so there is absolutely no reason to waste resources on doing this database lookups.

another concern is that this is not robust, if tortoise doesn't have dependency when dependant object is processed - it will fail validation and it will retry hopefully in some future.

to address both of those concern component that maintain state should be able to provide information about missing objects (for example as an error), and fetcher should do request based on missing object, regardless of that is stored in the database. after fetching complete validation should be re-done. in other words it should be up to handler to check that data is available, some handlers may do it using database, but in some places we already have all necessary data in memory.

@countvonzero

[dshulyak]