Upgrade wallet format to version 2 for security

[yilunzhang]

We recently proposed the next wallet format (v2) that is much safer than the current version (v1): https://forum.nkn.org/t/nkp-0020-upgrade-wallet-format-v2/2205

We have created a PR to support v2 format in nknd/nknc, and will also upgrade go/js sdk shortly.