Check for forks less frequently and keep track of the highest level verified from the detector

[ivanopagano]

To store the latest level that the detector checked in previous cycles and when the next detection check is done, verify all the blocks in between...

This complicates a bit this phase and carries additional risks to be handled.

@vishakh: I'd like your opinion on how to best proceed. I'd suggest going on with this (strictly speaking: incorrect) solution, and add the corrected checking as part of the next train of changes related to handling missing cases of invalidation (i.e. big-maps handling). But you might want to wait and handle this first and now.

Originally posted by @ivanopagano in https://github.com/Cryptonomic/Conseil/pull/903#discussion_r495914009