Interactive session should not spin waiting for statement result

Currently, when submitting statements for execution, the session code does this:

    val future = Future {
      val id = client.submitReplCode(content.code)
      info(s"Running statement $id: ${content.code}")
      waitForStatement(id)
    }

waitForStatement calls itself recursively until the statement is finished; that means that a thread in the global pool will be blocked until that statement finishes, which is bad and limits how many statements can be submitted concurrently.

We either should make the polling a recurring task (instead of a blocking task), or make the monitoring asynchronous.