RFR: Fix parallel object-monitor scanning

[Roman Kennke]

I observed an assert being thrown in the parallel object-monitor
scanner, and it happens because we don't always scan the object-monitor 
roots. The ShenandoahRootProcessor for initial-evacuation will not use
it, and therefore not find any CLAIMED blocks.

This relaxes the assert.

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

Ok?

Roman