v2.1: Store epoch in MaxAge (backport of #3485)

[mergify]

Problem

• MaxAge stores the last slot in the epoch which is confusing

Summary of Changes

• Store Epoch instead

Fixes #

---

This is an automatic backport of pull request #3485 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of 5a6f518 has failed:

On branch mergify/bp/v2.1/pr-3485
Your branch is up to date with 'origin/v2.1'.

You are currently cherry-picking commit 5a6f518c60.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Changes to be committed:
	modified:   core/src/banking_stage/consume_worker.rs
	modified:   core/src/banking_stage/consumer.rs
	modified:   core/src/banking_stage/transaction_scheduler/prio_graph_scheduler.rs
	modified:   core/src/banking_stage/transaction_scheduler/scheduler_controller.rs

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   core/src/banking_stage/scheduler_messages.rs
	both modified:   core/src/banking_stage/transaction_scheduler/transaction_state.rs
	both modified:   core/src/banking_stage/transaction_scheduler/transaction_state_container.rs

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally