Improvement of the HTTP Fetcher and Queue when status 500/503 is returned

When the Fetcher tries to access an URI and gets status 500/503 is returned, the URI is removed when the response (empty list of triples) is sent to the Frontier.
The Fetcher should implement the Retry Pattern. A parameter should be passed to the frontier to determine
how many retries should execute and the waiting time between retries. If still, a flag should be put in Crawleable URI data map, so the queue won't delete this URI and may crawl it later.

Start by tracing the Fetcher path for HTTP 500/503 responses through the Frontier and the queue's Crawlable URI data map. Confirm how empty triple responses currently remove URIs, then define completion as configurable retry counts and wait times, with failed URIs retained for later crawling after retries are exhausted.

Written by the indexing model from the issue text.