Reuse staking keys from `gen_keys`

[shenkeyao]

We sometimes call gen_keys multiple times to get the staking keys. We can instead reuse the first key generation result to avoid redundant computation.

E.g., in the minimal_test_network function, we first use gen_keys to get the staking keys pub_keys, then for each validator, we call the genesis function which also calls gen_keys to get staking keys known_nodes. known_nodes and pub_keys are the same, so we can just pass pub_keys as an input to genesis.