Adjust create account fee in Sharding

[NIC619]

What was wrong?

Since

1. FlatTrieBackend
2. Binary Trie

is adopted in Sharding, touch_account doesn't seem to be needed because account's code_hash, balance and nonce now belongs to different trie entries. Also, initializing these attributes to b'' has no effect in a Binary Trie.

How was it fixed?

• remove touch_account called in ShardingComputation.apply_message solved in Fix transaction execution #313
• fix gas fee charged for creating a new account(to be discussed, see below)

However since we no longer create a new account like we used to, we should not charge create_gas_fee but instead (1)separately charge for setting code_hash, nonce and balance if not been set before. Or (2)we still charge it but multiply it by x(say 3 since there will be 3 trie entries for one account, but I'm not sure that's all the reason it's charged 25000 gas for creating a new account).

At first thought there doesn't seem to be an easy and clean way to charge separately so I personally would prefer (2).

Cute Animal Picture

[NIC619]

Rebased and seems like touch_account is removed in #313 . This PR is adjusted to change the fee charged for create_gas_fee.

[vbuterin]

However since we no longer create a new account like we used to, we should not charge create_gas_fee but instead (1)separately charge for setting code_hash, nonce and balance if not been set before

IMO bad idea. The reason is that the code hash, node and balance still share ~90% of their Merkle tree branch, so it's most practical to consider it as a single item. So I'd prefer keeping the cost of 25000 for state changes that make accounts nonempty for the first time.

[NIC619]

Then I think this one is good to go.