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/client/client.rs

@@ -2381,14 +2381,13 @@ impl ProvingBlockChainClient for Client {
 		env_info.gas_limit = transaction.gas.clone();
 		let mut jdb = self.state_db.read().journal_db().boxed_clone();
 
-		state::prove_transaction(
+		state::prove_transaction_virtual(
 			jdb.as_hashdb_mut(),
 			header.state_root().clone(),
 			&transaction,
 			self.engine.machine(),
 			&env_info,
 			self.factories.clone(),
-			false,
 		)
 	}