RFR: 8236873: Worker has a deadlock bug

[Daniil Titov]

Please review a change [1] that fixes a deadlock issue [2] in sun.tools.jconsole.Worker class.

There is no need in guarding "stopped" flag by a lock. The fix removes this excessive locking and
instead makes the flag volatile.

Mach5 tier1-tier3 tests passed.

[1] Webrev : http://cr.openjdk.java.net/~dtitov/8236873/webrev.01/
[2] Issue: https://bugs.openjdk.java.net/browse/JDK-8236873 

Best regards,
Daniil