RFR: 8158946 - btree009 fails with assert(s > 0) failed: Bad size calculated

[Derek White]

https://bugs.openjdk.java.net/browse/JDK-8158946

This bug is a race between an assertion in the CMS sweeper and a Java 
thread loading a class. See the bug report for full description of the race.

The fix is to bypass the assertion in CMS for this case, and add extra 
assertions to catch similar data races in the future.

Webrev: http://cr.openjdk.java.net/~drwhite/8158946/webrev.01

Tests: jprt

- Derek