Replace local highest_finalized_slot with pool.highest_finalized_slot.

[wen-coding]

Currently highest_finalized_slot is just used to:

1. check whether the returned highest_finalized_slot is larger than the local copy, which certificate_pool already checks before returning
2. used in Standstill event creation, here we can actually get the value from cert_pool.highest_finalized_slot().

So we can remove the local highest_finalized_slot, makes the code a bit simpler.