RFR(11u): 8257621: JFR StringPool misses cached items across consecutive recordings

[Roman Kennke]

Any takers? This is becoming relevant for some Graal work... :-)

Cheers,
Roman

> Can I please get a review of the following backport:
> 
> Original issue:
> https://bugs.openjdk.java.net/browse/JDK-8257621
> 
> The original fix:
> https://github.com/openjdk/jdk16/commit/7aac4dc1
> 
> JDK11u webrev:
> http://cr.openjdk.java.net/~rkennke/JDK-8257621-jdk11/webrev.00/
> 
> None of the code that has been changed to use jfrSignal in jdk16 seems 
> to exist in jdk11, so I left those parts out. I am not totally sure if 
> JfrStringPool::add() is correct.
> 
> I successfully ran JFR tests, including TestReEnableName.java.
> 
> Thanks,
> Roman