RFR: Traversal: Initialize extra liveness arrays when conc-threads > par-threads

[Roman Kennke]

Yesterday, Zhengyu discovered why sometimes we get liveness counters
off when running with conc-threads > par-threads. We only initialize
liveness arrays in init-traversal, and we do this in the workers, so
only the active workers get their liveness array initialized. Duh.

This fix checks in conc-traversal worker if the worker_id is >= the
last number of stw workers used, and if so, initializes its own
liveness array.

Test: hotspot_gc_shenandoah

http://cr.openjdk.java.net/~rkennke/init-extra-liveness/webrev.00/

Ok?

Roman