RFR(S) 8021335: Missing synchronization when reading counters for live threads and peak thread count

[dean.long at oracle.com]

https://bugs.openjdk.java.net/browse/JDK-8021335
http://cr.openjdk.java.net/~dlong/8021335/webrev.3/

This change attempts to fix an old and intermittent problem with 
ThreadMXBean thread counts.
Changes have 3 main parts:
1. Make sure hidden threads don't affect counts (even when exiting)
2. Fix race reading counts using atomic counters
3. Remove some workarounds in test (because they hide bugs)

dl