Is there any way to execute pure queries in auto-commit mode?

Exposed is great!

But it seems any operation in Exposed must be called within a `transaction` block. Sometimes it might be unnecessary to to use a real native transaction because the operations in it are pure queries.

```kotlin
val jamesList = transaction {
Users.select { Users.firstName eq "James" }
}
```

I've read the source code in [TreadLocalTransactionManager.kt](https://github.com/JetBrains/Exposed/blob/master/src/main/kotlin/org/jetbrains/exposed/sql/transactions/ThreadLocalTransactionManager.kt) and found that the `autoCommit` property is set to false after connection established.

In my option, it's better to provide a `execAutoCommit` method(or any other name) to encapsulate pure query operations, so that we can run queries in auto-commit mode, which might be good to performance in some cases.

```kotlin
val jamesList = execAutoCommit {
Users.select { Users.firstName eq "James" }
}
```

Anyway, thanks for your contribution to this framework, I like it very much. Please consider adding this feature.