Synchronize preemptionTimer creation
Synchronize preemptionTimer creation

The preemptiontimer needs to be initialized before `post()`ing lambdas
on the io_thread or later lambdas may be started with a null
preemptionTimer.
This is a bit weird from an ownership point of view, but seems to keep
TSAN happy.

mozart/mozart2,mozart/mozart2,mozart/mozart2