Crawling strategy interface improvements

[sibiryakov]

Handling of scoring log events in strategy worker was refactored. The major changes are:

• FrontierManager is passed to strategy class,
• MessageBusScheduler is introduced to buffer update_score events during strategy worker execution.
• StatesContext is introduced to allow proper creation of Frontera request object (with instant state retrieval from storage) using BaseCrawlingStrategy.create_request(url) method.
• Scheduling is done using BaseCrawlingStrategy.schedule(request) method. Previous mechanism is eliminated.