test: add binary e2e test for joining with an outdated checkpoint

[matthias-wright]

Changes:

• Adds a route to the RPC server that returns the current epoch
• Adds another binary e2e test (similar to stake-and-checkpoint), where a node joins with an outdated checkpoint (2 epochs old). Since the nodes keep track of 4 peer sets by default, a node should be able to join with a checkpoint that contains an outdated peer set, as long as it is not older than 4 epochs.