RFR (S) 8230003: Make Monitor inherit from Mutex

[coleen.phillimore at oracle.com]

Summary: Reverse inheritance that makes more sense.

See bug for more description.  Tested with hs-tier1-3 with 
linux-x64-debug and hs-tier1 on Oracle platforms.

open webrev at http://cr.openjdk.java.net/~coleenp/2019/8230003.01/webrev
bug link https://bugs.openjdk.java.net/browse/JDK-8230003

Thanks,
Coleen


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