Optimized initial user account balance

[JMLX42]

Today, the root "authority" blockchain accounts credits the user browser-side generated account with more than enough ETH to perform the smart contract method call.
We should credit only the minimum amount of ETH necessary to handle the call transaction.

With this modification, the balance of the user account should be 0.
If it's not zero, the balance should be sent back to the root account.