Berkeley_migration app should be able to migrate from any canonical block in archive database

[ghost-not-in-the-shell]

Currently berkeley_migration app always migrated all the canonical blocks in the archive database. If we want to pick a canonical block (which is not the top most canonical block) as the fork block, then this would be problematic.

Changes are needed in berkeley_migration app and berkeley_migration_verifier (but not in replayer) app to allow people migrate from any block on the canonical chain.