Use an atomic increment for nr_times_visited

[benkonrath]

This update uses an update query with an F expression to atomically increment nr_times_visited. The update query is run using to transaction.on_commit to ensure that the value is only incremented when the current transaction succeeds (transaction.on_commit calls the function immediately if there is no transaction).

I also set DEFAULT_AUTO_FIELD to BigAutoField to silence the warning when running the tests.

Feedback is appreciated. Feel free to also edit the PR if there are changes you want to make.

[alextreme]

Thanks Ben, appreciate the effort! This is of course a better implementation than what I did years back

[benkonrath]

I think some of this stuff didn't exist in Django back then. :-) Thanks for maintaining this app all these years!