RFR: 8170299: Debugger does not stop inside the low memory notifications code

[Daniil Titov]

Hello,

Please review a new version of the change [1]  that fixes the problem with the  debugger not stopping in the low memory notification code. The fix moves the send notifications task from
not visible ServiceThread to a new visible NotificationThread. This version of the  change also introduces  a new VM option to opt-out from the new behavior.

Previous email threads:
https://mail.openjdk.java.net/pipermail/serviceability-dev/2019-August/028863.html 
https://mail.openjdk.java.net/pipermail/serviceability-dev/2019-July/028608.html 

The proposed CSR [3] is for adding  a new VM option UseNotificationThread  (default true) to opt-out from the new behavior. 

Testing: Mach5 tests tier1, tier2, tier3, and tier7 successfully passed.

[1] Webrev:  http://cr.openjdk.java.net/~dtitov/8170299/webrev.05/
[2] Bug: https://bugs.openjdk.java.net/browse/JDK-8170299 
[3] CSR: https://bugs.openjdk.java.net/browse/JDK-8231593 

Thanks,
Daniil