RFR: 8298526: JFR: Generate missing filename for time-bound recordings

[Erik Gahlin]

Could I have a review of a change that generates a filename for a recording where a duration has been specified, but no filename has been set. This is most likely what the user wants. This behavior also prevents stopped recordings from hanging around without being closed. 

For the few users, if any, that want a time-bound recording without a dump file can do filename=/dev/null.

Testing: jdk/jdk/jfr

Thanks
Erik

-------------

Commit messages:
 - Fix comment
 - Initial

Changes: https://git.openjdk.org/jdk/pull/11721/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11721&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298526
  Stats: 38 lines in 2 files changed: 24 ins; 0 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/11721.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11721/head:pull/11721

PR: https://git.openjdk.org/jdk/pull/11721