Update nonce counter/bitfield sizes

[lrettig]

api/proto/spacemesh/v1/types.proto

Lines 64 to 67 in 6dbb77d

	message Nonce {
	uint64 counter = 1;
	uint32 bitfield = 2; // this is actually limited by uint8, but no type for that.
	}

Update to match the size that's finalized in spacemeshos/pm#104