Remove fixed fee

[ureeves]

We substitute the spend function in the transfer contract for a spend_and_execute function, that also executes the contract call.

This fixes a bug where it was impossible to inform a genesis contract about whether they're being called in "management mode" by the VM - leading to sensitive functions being reachable by any contract call.

It also removes the fixed fee per transfer, since it was extremely artificial, and the regular transfer fee, while variable, is not much higher regardless.

[HDauven]

LGTM, nice cleanup!

[moCello]

LGTM, only a few nits in the comments

[herr-seppia]

As per discussion, tests should be added to verify consistency between receipt.gas_spent and wallet.balance

Apart from that, I've currently RFC due to a regression found with the following manual test:

1. Run a local node
2. Send a normal transfer with a very low gas (10k point are enough)
3. Node is not able to create blocks due to hash mismatch between EST and VST

Tried the same with master, and transaction is properly discarded by EST and removed from the mempool

See also #1134

[HDauven]

LGTM, that test is a nice addition. :)

[ureeves]

As per discussion, tests should be added to verify consistency between receipt.gas_spent and wallet.balance

Apart from that, I've currently RFC due to a regression found with the following manual test:

1. Run a local node

2. Send a normal transfer with a very low gas (10k point are enough)

3. Node is not able to create blocks due to hash mismatch between EST and VST

Tried the same with master, and transaction is properly discarded by EST and removed from the mempool

See also #1134

This has been addressed, and the fix is present in the latest master together with the a test that ensures it will never happen again.

[herr-seppia]

LGTM

But I still believe that having a fixed fee for transfer would be a benefit for the network. Does it worth to reintroduce it?