IF: Implement finalizer policy change at appropriate time

[greg7mdp]

Resolves #1618.

This initial implementation requires two 3-chains for a new finalizer_policy to become active.

This updates the finalizer_policies flat_map to store a state [proposed, pending] and a finalizer_policy associated to a block_number:

block_num => state, finalizer_policy

When state == proposed, the block_num identifies the block in which the new policy was proposed via set_finalizers.
when that block becomes final, according to the block_header_state's finality_core,

1. the policy becomes pending
2. its key block_num, in the proposer_policies map, is the current block

When this current block itself becomes final, the policy becomes active.

[ericpassmore]

Note:start
group: IF
category: INTERNALS
summary: Improve logic for activation of finalization policy. Now it activates the finalizer policy only after critical blocks become final.
Note:end