``try_number`` is no longer incremented during task execution

Previously, the try number (``try_number``) was incremented at the beginning of task execution on the worker. This was problematic for many reasons. For one it meant that the try number was incremented when it was not supposed to, namely when resuming from reschedule or deferral. And it also resulted in the try number being "wrong" when the task had not yet started. The workarounds for these two issues caused a lot of confusion.

Now, instead, the try number for a task run is determined at the time the task is scheduled, and does not change in flight, and it is never decremented. So after the task runs, the observed try number remains the same as it was when the task was running; only when there is a "new try" will the try number be incremented again.

One consequence of this change is, if users were "manually" running tasks (e.g. by calling ``ti.run()`` directly, or command line ``airflow tasks run``), try number will no longer be incremented. Airflow assumes that tasks are always run after being scheduled by the scheduler, so we do not regard this as a breaking change.