Fix race between G1PeriodicGCTask checks and GC request

The G1PeriodicGCTask performs various checks to determine whether a periodic GC should be performed. Even if all those tests are performed in a context where a GC cannot be running concurrently (JDK-8271215, executing the checks in an STS-joiner), there is still a race between the end of that checking and the actual request via try_collect. This can be addressed by capturing the GC counters as part of that checking context and passing them to try_collect for use by the VMOp.