Minimum delay based on download finish time

[aecio]

• Modified PolitenessScheduler to compute the delay between same-domain requests based on the time when the download finished
• Refactoring of FethcedResultHandler to simply notify the LinkStorage that the download finished and to delegate data processing to other handlers for the appropriate link type
• Fixed unit and integration tests to consider the new scheduler workflow that requires notification when the download was finished

Fixes #179.