Add unique id to collections in addition to names

[Rulexec]

We can use them internally to be sure that async task is belongs exactly for specified collection, not already deleted one.

[Rulexec]

Forbid @ symbol in collection name, allow collection_name@42 ids in requests to specify concrete collection instance.

Always use this id in readers. Then we can not delete collections physically, just mark them as deleted by storing collection_name -> Option<active_id>. If it's None, collection with such name was deleted.