apps/blehr: shared variable safety

https://github.com/apache/mynewt-nimble/blob/7b5b5e5b512133e50ef8a517b13e7269f9c821fd/apps/blehr/src/main.c#L33

Probably not an actual bug, but shouldn't `notify_state` in principle use an `atomic_bool` or another locking mechanism?

It is written in `blehr_gap_event` callback while it is read in the `blehr_tx_hrate` from a timer task.

Start in apps/blehr/src/main.c around line 33, then trace notify_state access in blehr_gap_event and blehr_tx_hrate. Check the Mynewt execution model for interactions between the callback and timer task, and determine whether the shared access requires synchronization. Done means the safety concern has a justified resolution in the application.

Written by the indexing model from the issue text.