RFR: 8234888: EventStream::close doesn't abort streaming thread

[Erik Gahlin]

Hi,

Could I have a review of a fix that makes sure the streaming thread is 
terminated when an EventStream is closed.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8234888

Webrev:
http://cr.openjdk.java.net/~egahlin/8234888/

Testing: jdk/jfr

Thanks
Erik