WIP: Incremental verify checkpoints

[ThomasBrady]

Description

Adds --trusted-hash-file argument to the verify-checkpoints command to support appending new verified checkpoints starting from the last checkpoint in the trusted hash file.

Removes --output-file argument. Now, users provide the path to an existing or to-be-created trusted hash file. If existing, the command will append new verified checkpoints to this file. If to-be-created, the command will write all verified checkpoints since genesis to the file.

Design doc: https://docs.google.com/document/d/1GRzHAO4_YrfanXqoVc1UDIMhUV10PFqIMQyOxlPOW_s/edit

Resolves #X

Checklist

• [ ] Reviewed the contributing document
• [ ] Rebased on top of master (no merge commits)
• [ ] Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• [ ] Compiles
• [ ] Ran all tests
• [ ] If change impacts performance, include supporting evidence per the performance document