large delay when using start_periodic_saving

[jbweston]

The current implementation of 'start_periodic_saving' does await sleep(interval):

https://github.com/python-adaptive/adaptive/blob/88f02588cf66d0276b9b77da466aa71c7c1f1773/adaptive/runner.py#L839C1-L843C50

This means that there can be a large interval-length delay between the time the runner completes and the time that the last write is made by start_periodic_saving.

This is very confusing when you wait for the runner to finish with await runner.task and the results are not yet written.

Fix is to have the await await both the timeout and runner.task (remembering to shield runner.task)