Remove default TxBoundary typeclass instances

In the following case, DB connection will be uselessly grasped by localTx's `TxBoundary` control if it's enabled by default since 2.2.

```scala
DB.localTx { implicit s =>
  // works with the DB connection
  ...
  doFutureOperation()
}

def doFutureOperation()(implicit ctx: ExecutionConttext): Future[Something] = {
  Future { 
    // Assumes this block sometimes takes long time due to other operations (e.g. external API call).
    // Although this code probably should be outside of this localTx block ideally,
    // the author of code might think it's safe because this Future ops won't join the transaction
  }
}
```

If there is a risk that existing applications may be in trouble, we should keep backward compatibility after 2.2 release too.