The way we store AccountData

When coming to store account data, there are two possibilities:

1. store it in the pallet that manages it (like pallet balances or duniter account in our case)
2. store it in system pallet

An example is given here: https://paritytech.github.io/substrate/master/pallet_balances/pallet/type.Account.html

In our case, we are using option 2. Questions:

• could we hide pallet balances storage item since it's not used?
• could we use the StoredMap implementation to retrieve the account data client side or does it requires to implement a custom rpc which is unwanted
• can this cause a problem on framework upgrades when try_mutate_exists implementation becomes too different from substrate expected implementation?

I just wanted to track this topic in an issue in case we meet problem with it at some point.

added S-needdiscussion label