esp_timer: do not allow deleting timers while callbacks are dispatched

esp_timer: do not allow deleting timers while callbacks are dispatched

timer_process_alarm function of esp_timer holds a spinlock for the
entire duration of its operation, except for the time when timer
callback function is called. It is possible that when
timer_process_alarm releases the spinlock, a higher priority task may
run and delete the timer. Then the execution will return to
timer_process_alarm, and this will either cause a crash, or undesired
execution of callback after the timer has been stopped or deleted.

To solve this problem, add a mutex which will prevent deletion of timers
while callbacks are being dispatched.