RFR(S): 8235206: JFR TestCrossProcessStreaming - validate that data can be consumed while it is being produced

[mikhailo.seledtsov at oracle.com]

Please review this change that improves the JFR 
TestCrossProcessStreaming test.

In order to test that the events can be consumed at the same time as 
they are produced by another process, I have
added a sequence of "produce first batch", wait for N number of events 
to be consumed in the first batch,
direct producer to produce the next batch. For full explanation of the 
sequence and detailed test description
see the head comment in the class.

I have also simplified and streamlined cross-process signaling, plus 
some refactoring and cleaned.

     JBS: https://bugs.openjdk.java.net/browse/JDK-8235206
     Webrev: http://cr.openjdk.java.net/~mseledtsov/8235206.00/
     Testing:
       - ran the test on several platforms - PASS
       - will run many times for stability - in progress


Thank you,
Misha