allow running `verify-checkpoints` incrementally

[marta-lokhova]

Currently, verify-checkpoints command always starts verification at genesis, and either stops at the most recent network ledger, or at a specified --history-ledger. The problem is verifying all the way back to genesis takes hours, and typically operators want to keep trusted hashes file up-to-date, so verifying the whole chain isn't efficient. We should design a solution to amortize the cost of trusted hash generation.

[ThomasBrady]

Design / planning doc https://docs.google.com/document/d/1GRzHAO4_YrfanXqoVc1UDIMhUV10PFqIMQyOxlPOW_s/edit?usp=sharing cc @marta-lokhova / @anupsdf