RFR (S) 8224847: gc/stress/TestReclaimStringsLeaksMemory.java fails with reserved greater than expected

[coleen.phillimore at oracle.com]

Summary: Rehash threshold was too low for StringTable, and rehashed size 
table was too large.

Tested with TestReclaimStringsLeaksMemory.java test 100x no failures, 
also tested with hs-tier1-3.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8224847.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8224847

Thanks,
Coleen