[11u] RFR (S) 8244205: HTTP/2 tunnel connections through proxy may be reused regardless of which proxy is selected

[Aleksey Shipilev]

Original bug:
   https://bugs.openjdk.java.net/browse/JDK-8244205
   https://hg.openjdk.java.net/jdk/jdk/rev/dd61d60329f6

This fixes the issue exposed by JDK-8244031 backport.

The patch applies nearly exactly, but the tests need adjustments for 11u:
   - jdk.test.lib.net.SimpleSSLContext is jdk.testlibrary.SimpleSSLContext
   - @library /test/lib is @library /lib/testlibrary

Testing: java/net/httpclient tests

-- 
Thanks,
-Aleksey