RFR: 8214295: Populate handlers while holding streamHandlerLock

[Seán Coffey]

JDK-8213942 synchronization fix can be improved. Details in bug report.

https://bugs.openjdk.java.net/browse/JDK-8214295

http://cr.openjdk.java.net/~coffeys/webrev.8214295/webrev/

--
Regards,
Sean.