VNs should sync with epoch before registration becomes active

[sdbondi]

Problem

Validators currently sync when joining the network on the epoch that they are active. This leads to leader failures as they are unable to propose until sync completes.

Proposal 1

A validator node should detect that they are going to be active in the next epoch and sync up to the current epoch. For simplicity (unless it turns out to be simple enough to keep in sync with the current epoch) it is acceptable if the final epoch is synced when the validator becomes active, as this should be much shorter than syncing from scratch