replay: Fix --loop to work when --singlethread is in effect

The original implementation of --loop was made only to the code
implementing multi-threaded replay, (the RelayRunner class and
friends), so that when --singlethread is in effect, the --loop option
has no effect.

This commit fixes the bug by copying the --loop logic to the
single-threaded case as well.

Thanks to Peter Lohrmann for reporting the bug.