JFR: At most one native periodic event thread at a time

Today two threads can call into JVM::emitEvent(long, long, long) at the same time. This can happen if a recording is started/stopped at the same time as a periodic event is emitted.

It's unlikely they will emit the same event as start/stop triggers event with "beginChunk", "endChunk" or "everyChunk", while periodic events have an interval, for example "1 s". Still, they might access the same data structure, for example Performance Counters on Windows.

There is no known issue because of this, but if if we see some strange intermittent failures go away, we may want to consider a backport to JDK 11 and JDK 17.