vumeter: call tmr_start() from re_main thread

- tmr_start() should not be called from a thread that is not running
  the re_main() loop. i.e. tmr_start() MUST be called from the re_main()
  loop thread, unless the call is wrapped by re_thread_enter/leave().

- the audio-processing handlers encode/decode are handling real-time
  audio samples, and should not be blocked.