Review Request: 8009298 and 8009777

[Zhengyu Gu]

8009298: NMT: Special version of class loading/unloading with runThese 
stresses out NMT <https://jbs.oracle.com/bugs/browse/JDK-8009298>
8009777: NMT: add new NMT dcmd to control auto shutdown option 
<https://jbs.oracle.com/bugs/browse/JDK-8009777>

The two are related. The change is to add an addition VM diagnostic 
command-line option "AutoShutdownNMT".

By default, native memory tracking runtime shuts itself down under 
stress situation, and release memory to allow JVM continue running. But 
this feature can be undesirable when this is the situation we really 
want to know what's happening to JVM memory usage, even that means that 
we need to sacrifice JVM performance.

When AutoShutdownNMT is set to false, NMT will try its best to stay 
alive by slowing down threads calling memory allocation/de-allocation.

Webrev: http://cr.openjdk.java.net/~zgu/8009298/webrev.00/

Tests:
The test case is special version of runThese, it uses 12 threads to 
load/unloading classes without executing any tests.

Following two memory usage graphs show the different behavior with 
AutoShutdownNMT on/off.

AutoShutdownNMT = true
   http://cr.openjdk.java.net/~zgu/8009298/webrev.00/autoShutdown.png
   The steep drop in memory usage was result of shutting down of native 
memory tracking.

AutoShutdownNMT = false
http://cr.openjdk.java.net/~zgu/8009298/webrev.00/noAutoShutdown.png
    The steep dig was result of class unloading.


Thanks,

-Zhengyu