fix (light/provider) : Make read_only executions read-only (#9591)

* `ExecutionsRequest` from light-clients as read-only

This changes so all `ExecutionRequests` from light-clients are executed
as read-only which the `virtual``flag == true ensures.

This boost up the current transaction to always succeed

Note, this only affects `eth_estimateGas` and `eth_call` AFAIK.

* grumbles(revert renaming) : TransactionProof

* grumbles(trace) : remove incorrect trace

* grumbles(state/prove_tx) : explicit `virt`

Remove the boolean flag to determine that a `state::prove_transaction`
whether it should be executed in a virtual context or not.

Because of that also rename the function to
`state::prove_transction_virtual` to make more clear

ethcore/src/state/mod.rs

@@ -224,17 +224,16 @@ pub fn check_proof(
 	}
 }
 
-/// Prove a transaction on the given state.
+/// Prove a `virtual` transaction on the given state.
 /// Returns `None` when the transacion could not be proved,
 /// and a proof otherwise.
-pub fn prove_transaction<H: AsHashDB<KeccakHasher> + Send + Sync>(
+pub fn prove_transaction_virtual<H: AsHashDB<KeccakHasher> + Send + Sync>(
 	db: H,
 	root: H256,
 	transaction: &SignedTransaction,
 	machine: &Machine,
 	env_info: &EnvInfo,
 	factories: Factories,
-	virt: bool,
 ) -> Option<(Bytes, Vec<DBValue>)> {
 	use self::backend::Proving;
 
@@ -252,7 +251,7 @@ pub fn prove_transaction<H: AsHashDB<KeccakHasher> + Send + Sync>(
 	};
 
 	let options = TransactOptions::with_no_tracing().dont_check_nonce().save_output_from_contract();
-	match state.execute(env_info, machine, transaction, options, virt) {
+	match state.execute(env_info, machine, transaction, options, true) {
 		Err(ExecutionError::Internal(_)) => None,
 		Err(e) => {
 			trace!(target: "state", "Proved call failed: {}", e);