BUG/MEDIUM: threads: fix the no-thread case after the change to the sync point

BUG/MEDIUM: threads: fix the no-thread case after the change to the sync point

In commit 0c026f4 ("MINOR: threads: add more consistency between certain
variables in no-thread case"), we ensured that we don't have all_threads_mask
zeroed anymore. But one test was missed for the write() to the sync pipe.
This results in a situation where when running single-threaded, once a
server status changes, a wake-up message is written to the pipe and never
consumed, showing a 100% CPU usage.

No backport is needed.