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 like the addition of `in_flight` but there's no need to stash `_nothing_in_flight` inside of `wait_for_ajax`. Mind putting it back where it was, or was that necessary for something?