[15] RFR 8236878: Use atomic instruction to update StringDedupTable's entries and entries_removed counters

Hi, Please review this small change that uses atomic operations to update StringDedupTable's entries and entries_removed counter. This is *not* a correctness fix or performance enhancement, but for Shenandoah GC to move StringDedupTable cleanup task into concurrent phase, while holding StringDedupTable_lock. Bug: https://bugs.openjdk.java.net/browse/JDK-8236878 Webrev: http://cr.openjdk.java.net/~zgu/JDK-8236878/webrev.00/index.html Test: hotspot_gc (fastdebug and release) on x86_64 Linux Submit test in progress. Thanks, -Zhengyu