Change can_send_validator_set_update logic

[sug0]

Related with #599

Instead of sending a validator set update at the first block of an epoch, we should do this at the second block of an epoch. This is because we don't know, in advance, the first block of a new epoch until we call finalize_block(). The only exception is, of course, at genesis, where we know the height of the first block.

This change only affects ABCI++ builds of the ledger.