Define _TASK_THREAD_SAFE for TaskScheduler

[schlimmchen]

I appreciate the use of TaskScheduler as an alternative to extending the loop() approach.

To make further use of TaskScheduler's features, it makes sense to me to change task scheduling intervals dynamically at runtime. Example: The MQTT loop is executed at a specific interval, the MQTT publish interval. Once the user changes this interval, the scheduling interval of the MQTT loop should be changed accordingly. The straight-forward approach is to call setInterval() in the respective web server handler. That will execute in the web server's context. Changing TaskScheduler task properties from another context is not inherently save.

In platformio.ini, the build_flags should use -D_TASK_THREAD_SAFE to enable the use of an internal mutex which prevents unexpected behavior once multiple threads (FreeRTOS tasks) start changing task or scheduler settings.

An excerpt from the TaskScheduler docs:

#define _TASK_THREAD_SAFE

…will use an internal mutex to protect task scheduling methods from preemption and unexpected behavior. This is a recommended option for esp32 and/or other MCU's running TaskScheduler under preemptive scheduler like FreeRTOS.

The author suggests to define this flag when using TaskScheduler on ESP32.

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