Smarter retries strategies: same region and backoff

The current retry strategy for all checks is fairly limited. We want to expand the possibilities for retrying with two options:

- same region: right now, retries are always scheduled on a different region than the original attempt. This is not ideal for geo-fenced checks or region-aware websites rendering different content for different regions.

- backoff strategies: we want to enable multiple retries according to a strategy the user can define: linear or exponential so the target system has more time to recover from an initial failure.