Add a `--delete-duplicates` flag to `graphman chain check-blocks` command

[tilacog]

Do you want to request a feature or report a bug? Feature.

What is the current behavior? graphman chain check-blocks panics if it finds more than a single block for a given number in the block cache.

What is the expected behavior? This command should provide a flag to automatically delete those duplicated blocks in the block cache instead of exiting with an error.