Crawl active stocks & options more often than inactive ones

[Tyruiop]

Currently all options are crawled at the same rate, however some options have little to no variation over extended periods of time while others change rapidly during the day.

• We need a heuristic to detect when a [ticker, expiration date] pair becomes active and high
• We need the crawler to still provide coverage guarantees (e.g. make sure that all such pairs are crawled at least once a day).