RFR (S): 8140600: Convert unnecessarily malloc'd Monitors to value members

[Derek White]

This small fix changes a malloc'd monitor into an embedded monitor. Also 
adds assert that no one holds the monitor when we destruct it.

*Bug*: https://bugs.openjdk.java.net/browse/JDK-8140600
*Webrev*: http://cr.openjdk.java.net/~drwhite/8140600/webrev.01/
*Tests*: jprt

Note: Merged with mgerdin's change to g1YoungRemSetSamplingThread.

Thanks!

  - Derek
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