RFR: Refactor concurrent partial shortcut

[Aleksey Shipilev]

http://cr.openjdk.java.net/~shade/shenandoah/concpartial-shortcut/webrev.01/

This rebuilds the has_work shortcut, so it does not mess up with code paths in conc-partial and
final-partial. It also moves the in_progress=true flag setting to appropriate place, so that we
don't go out via the shortcut, and have this flag still set.

Testing: hotspot_gc_shenandoah (+ hand tests for shortcut)

Thanks,
-Aleksey