RFR: Don't attempt to leave STS in final-traversal, where we never enter STS

[Roman Kennke]

In final-traversal we may enter STSLeaver through
check_and_handle_cancelled_gc() even though we never entered the STS
scope (it's final-traversal after all). This changes the call to
STSLeaver only during conc-traversal:

http://cr.openjdk.java.net/~rkennke/stsyield/webrev.00/

Testing: tier3_gc_shenandoah

OK?
Roman