Separate key, account, and id generation into its own library

[shahankhatch]

Instead of depending on geth to create the accounts, it would be nice to separate nodekey.go and the account generation code into their own library; the library can then be reused for other apps and can also be called from within an enclave.

Account generation code using geth is visible at: https://github.com/ConsenSys/quorum-deployer/blob/1269aa036be92401c964232d1219c8f9d8ca5df2/02-deploy/01-prepare-assets/address#L6