Support importing v2 snapshot even when global_property_object::proposed_schedule is not empty

[arhag]

PR #7404 introduced the WTMSIG_BLOCK_SIGNATURES protocol feature and in doing so required changes to the global_property_object. The intention of the changes in that PR were to still successfully import old serialized global_property_objects in v2 snapshots and upconvert as necessary to store as the new global_property_object in the state database. However, the code path to handle the legacy version in v2 snapshots does not actually exist. The reason it is still possible to import v2 most of the time (and indeed possible in the tests that explicitly test this) is due to the fact that the serialization of global_property_object in the new and old versions are the same if proposed_schedule.producers is the empty vector.

Code to explicitly add support for importing in the old global_property_object from v2 snapshots needs to be added to controller_impl::read_from_snapshot which will upconvert legacy:: producer_schedule_type to producer_authority_schedule (similar to how it is done already for the block_state). Furthermore, a test should be added (or existing one modified) to verify that import of a v2 snapshot is successful if the proposed_schedule.producers is not empty.

[arhag]

Resolved by #7717.