Add endpoint to chain_api that reports active (and pending if different) finalizer policies and each finalizers last tracked vote

[arhag]

Any history tracked by the chain plugin should be ephemeral (lost when nodeos is shut down). Therefore, there is no guarantee that last tracked vote information will be available for every finalizer in the active (and pending) policy within any particular nodeos instance. So the structure containing the last tracked vote information for each finalizers should be optional; returning nullopt for a particular finalizer does not necessarily mean that the finalizer has not voted yet, it just means this nodeos instance does not know what that finalizer's last tracked vote is.

The last tracked vote associated with each finalizer (if present) should include the timestamp, block ID, and block number of that block it last voted on, and it should also indicate whether that vote was strong or weak and the generation number of the policy it voted as part of (pick the pending over the active if the finalizer was present in both).