system:clang:tsan runtime doubled after switching to liburcu-qsbr

After merging the liburcu-qsbr changes, the total runtime of system:clang:tsan increased to 42 minutes up from around 19 minutes that it used to take before.

The runtime of a few tests increased by 3x-4x:

              runtime      with TSAN
rpz               4m            16m
rpzrecurse      6.5m            18m
kasp              3m             9m
rrsetorder        1m           4.5m

Side note: The increased runtime also unearthed some timing issues, making some tests fail more frequently in system:clang:tsan - notably, the upforwd test (which should be fixed once #4072 (closed) is resolved) and rrl.