cmd/cored: fix default BLOCK_KEY
cmd/cored: fix default BLOCK_KEY

The switch from ECDSA to ED25519 (chain/chainprv#1087) requires a new default
private key for the generator.

Closes chain/chainprv#1172.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@bobg




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain