RFR 8210476: sun/security/mscapi/PrngSlow.java fails with Still too slow

[Weijun Wang]

Please take a review at

   https://cr.openjdk.java.net/~weijun/8210476/webrev.00/

Before this fix, every PRNG::nextBytes calls all of CryptAcquireContext, CryptGenRandom, and CryptReleaseContext. Now, CryptAcquireContext is called once in PRNG::new, and CryptReleaseContext is called by a Cleaner, and nextBytes only calls CryptGenRandom.

I haven't read about thread-safety in any MS document, the current Windows-PRNG service is marked ThreadSafe=true (in SunMSCAPI.java). If we cannot be really sure, we can change it to false.

I've downloaded nearly 1000 Mach5 runs of this test, the enhancement is so good that I adjusted the test to be stricter.

	Before	After
	------	-----
Count	897	74
Min	0.38	0.008
Ave	0.97	0.011
Max	5.81	0.021

Please advise me if the following usage of Cleaner is correct because I really haven't observed the releaseContext method being called.

+        Cleaner.create().register(this,
+                () -> releaseContext(ctxt));

Thanks
Max