RFR (XS) 8215221: Serial GC misreports young GC time

[Aleksey Shipilev]

Bug:
  https://bugs.openjdk.java.net/browse/JDK-8215221

One of our guys pointed out the oddity in Serial GC logs: "Pause Young" includes the timing for Full
collection, and wrecks up GCId order as well. This is because GCTraceTime-s are nested on those
paths, and putting them in proper order, closer to collect_generation, fixes it.

The downside is that some of the cleanup actions are now outside the GCTraceTime. Those look
innocuous to omit to me, though.

Fix:
  http://cr.openjdk.java.net/~shade/8215221/webrev.01/

Testing: hotspot tier1, eyeballing GC logs, jdk-submit (running)

Thanks,
-Aleksey

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