Boosting priority of resources in indexing queue

Asynchronous indexing (#23) will allow a user to continue working while she has some resources in indexing queue. This means, that some of resources can have modified tags and unknown (yet) id. Such resources should gain priority of indexing.

Start by reading the asynchronous indexing work referenced in issue #23 and trace how resources enter and leave the indexing queue. Determine how resources with modified tags or an unknown id are identified; done means those resources are indexed before lower-priority queued resources while preserving normal queue behavior.

Written by the indexing model from the issue text.