[15] RFR 8250841: Shenandoah: need to reset/finish dead counters for StringTable/ResolvedMethodTable during STW root processing

[Zhengyu Gu]

Please review this small patch that triggers 
StringTable/ResolvedMethodTable cleaning and resizing after STW processing.

Without this fix, dead entries may accumulate in the tables, result long 
pause time.


Bug: https://bugs.openjdk.java.net/browse/JDK-8250841
Webrev: http://cr.openjdk.java.net/~zgu/JDK-8250841/webrev.00/

Test:
   hotspot_gc_shenandoah

Thanks,

-Zhengyu