Move mutex to CacheEntry (SiDetElementsRoadMaker_xk)

Move mutex to CacheEntry (SiDetElementsRoadMaker_xk class).

To reduce waiting time.

RunTier0Tests.py was fine. RunTier0Tests.log

for i in {0..37}; do athena --threads 10 SiSPSeededTrackFinder/SiSPSeededTracksStandaloneFromESD.py 2>&1 | tee ${i}.log; done

were all fine.

Tag @ssnyder and @amete