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

[Roman Kennke]

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