IF: Unification: IF transition of fork database usage

[arhag]

Support transition from reversible blocks in old fork database to blocks in new fork database. Convert block_header_state_legacy to block_header_state for the appropriate block. Handle transition at the appropriate time. Consider how a fork re-org would be handled in the middle of the transition.

Also handle any remaining small TODOs leftover from #1941 (e.g. in leap-util). leap-util todo fix moved to #2083 since not needed for testnet.

As soon as the transition begins, producer schedule changes will be rejected until the transition completes. The host function would not fail. It would just silently drop the new proposed producer schedules. This allows us to avoid needing to update proposer policy for the blocks created during the transition.

For this issue, we want a simpler (but unsafe) transition in which the block that first proposes the finalizer policy immediately becomes irreversible and atomically transitions from the old algorithm to the IF algorithm going forward.

This should also have a Boost test to show the transition works correctly.

[heifner]

• Needs to set active_finalizer_policy at transition.

[heifner]

Resolved by #2085