RFR: Parallel cleaning should process ResolvedMethodTable

[Aleksey Shipilev]

JDK 10 split out ResolvedMethodTable out of SystemDictionary. We need to clean
that up too, like G1 does.

Fix:
  http://cr.openjdk.java.net/~shade/shenandoah/rmt-clean/webrev.01/

Testing: hotspot_gc_shenandoah

Thanks,
-Aleksey