JFR: Generate missing filename for time-bound recordings

start jfr with duration
```
jcmd 31514 JFR.start duration=30s
31514:
Started recording 1.

Use jcmd 31514 JFR.stop name=1 filename=FILEPATH to copy recording data to file.
```


use JFR.check after 30s
```
jcmd 31514 JFR.check
31514:
Recording 1: name=1 duration=30s (stopped)
```

Unable to close the stopped jfr
```
jcmd 31514 JFR.stop name=1
31514:
Can't stop an already stopped recording.
```