Cleanup stake_ci uses, factor out multi-epoch leader

[akhinvasara-jumptrading]

• Clean up several 'uses' of stake_ci where it's not actually used.
• Implement fd_multi_epoch_leaders_t to handle stake updates and store/query leader schedules for multiple (currently assumed 2) epochs.
• Replace stake_ci with the new multi_epoch_leaders structure in the handful of cases we just care about leader schedule info, and not shred destinations
• minor refactors to use the new fd_stake_weight_msg_t struct to encode/decode stake update headers

To minimize blast radius, shred's fd_stake_ci is left unchanged at the cost of some code duplication, which ideally would be avoided by having stake_ci use the new multi_epoch_leader

Each consumer is modified in a separate commit, for easy reverts if needed. Review best done commit by commit, focusing on 60a75b9.