app itimer disabled after detach

I was testing #2037 (closed) by adding an itimer to the burst_threads test and the itimer disappeared after detach:

larm
alarm
alarm
pre-DR init
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
alarm
pre-DR start
pre-DR detach
pre-DR init
pre-DR start
pre-DR detach
pre-DR init
pre-DR start
pre-DR detach
pre-DR init
pre-DR start
pre-DR detach
all done