Introduce a semaphore to protect intermediate state from different RTR

Introduce a semaphore to protect intermediate state from different RTR
sessions to leak into the RDE via rtr_recalc. Only run rtr_recalc when
the last or only RTR session is done with the update. Run a new timer
along to ensure that the semaphore is not hold forever. The timeout
is currently a very generous 60sec, no RTR cache should be that slow.
OK tb@