Fix ajax logging

The `_thread_local.ajax_log_msg` (previous log message) var is not reset when there is a `TimedOutError` exceptions which manifests as random `Ajax done` lines without a preceding `Ajax running: something` lines in the cfme.log.
I put `_nothing_in_flight` into `wait_for_ajax` because `wait_for_ajax` manages (resets) the `_thread_local.ajax_log_msg` now, which ties them together.
If any other function were to use `_nothing_in_flight`, we'd have to make sure that it manages the `_thread_local.ajax_log_msg` as well.
So the main reason to put it into `wait_for_ajax` was to save us a potential headache.