Background indexing

It would be great, if a user could switch to other activities with the app while huge folders are being indexed.
E.g. trigger indexing of other folders or tag resources. It can happen that the user tags a resource, which is still not indexed.
Then we need to postpone modification of the storage, since we don't know the resource's id.
This means, beside of _indexing queue_ we also need to create _tagging queue_.

Start by tracing how the current indexing queue is triggered and how tagging modifies storage. Determine how indexing and tagging should proceed concurrently, including the case where a tagged resource has not received an id yet. Done means users can continue other activities, trigger additional indexing, and tag resources without losing or misordering storage updates.

Written by the indexing model from the issue text.