Correct Timeout rescheduling

Chrono changes "optimised" `Timeout::handler` in a way that broke users
who rescheduled the timeout during their attached callback.

Attempted optimisation is less necessary now that
`platform.callback-nontrivial` is set to false by default - that
setting reduces overhead of copying the `Callback` to almost nothing.

drivers/source/Timeout.cpp

@@ -27,11 +27,15 @@ namespace mbed {
 void TimeoutBase::handler()
 {
  if (_function) {
+ Callback<void()> function_to_call = _function;
+ // Clean up state to "detached" before calling callback; it may attach
+ // a new callback. Equivalent to detach(), but skips the remove();
+ // it's unnecessary because we're in the ticker's handler.
+ _function = nullptr;
  if (_lock_deepsleep) {
  sleep_manager_unlock_deep_sleep();
  }
- _function();
- _function = nullptr;
+ function_to_call();
  }
 }