RFR (XS): Enable -XX:+HeapDump{Before|After}FullGC

[Aleksey Shipilev]

Hi,

Little change: make Shenandoah dump the heap before/after Full GC, if requested,
like any diagnosable collector should do:
 http://cr.openjdk.java.net/~shade/shenandoah/heapdumps-before-after/webrev.01/

Thanks,
-Aleksey