Testnet fees

[grarco]

Closes #958

Implements a simplified fee system to mitigate DOS

[tzemanovic]

On a second thought, I think this will prevent from withdrawing from faucet, unless you already have some balance. Not sure if that's what we want

[grarco]

That's correct, at the moment this PR needs two things:

1. Define a sensible amount of the fee: it's currently at 100 micro NAMs but faucet releases up to 1000 NAM per request so we should probably raise this
2. Find a way to allow withdrawal from faucet

Regarding the second point, an idea could be this: in the three checks that we perform, when trying to access the balance of an account the first time the key is not in storage and we default to 0. For the testnet only we could change this default to be a minimum amount to allow the user to run the first transaction (ideally withdrawing from faucet)

[tzemanovic]

1. Define a sensible amount of the fee: it's currently at 100 micro NAMs but faucet releases up to 1000 NAM per request so we should probably raise this

With the faucet withdrawal PoW (#961), we can set faucet_pow_difficulty in genesis - it takes around a minute with difficulty 3 to solve. I'd say we can safely allow up to ~100 txs from that. Let me make the faucet limit configurable from genesis too, so we can control this with it and we can set it to 10_000 micro NAM.

2. Find a way to allow withdrawal from faucet

Regarding the second point, an idea could be this: in the three checks that we perform, when trying to access the balance of an account the first time the key is not in storage and we default to 0. For the testnet only we could change this default to be a minimum amount to allow the user to run the first transaction (ideally withdrawing from faucet)

Yeah, some along this line sounds good - I think we can introduce a "mainnet" feature, which we'll need for other things and allow faucet withdrawal with PoW without fees when not(feature = "mainnet")