RFR: 6766: Add heap usage before and after GC to the GC table

[Jean-Philippe Bempel]

Columns are hidden for now

Here some screenshots of the changes:

![JMC_UsedHeapBeforeAfterGC](https://user-images.githubusercontent.com/4610701/94465934-f030a300-01c0-11eb-8492-2b921f695833.png)



![JMC_UsedHeapBeforeAfterGCColumns](https://user-images.githubusercontent.com/4610701/94465962-fc1c6500-01c0-11eb-8dc9-5b3346668381.png)

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Commit messages:
 - Add heap usage before and after GC to the GC table

Changes: https://git.openjdk.java.net/jmc/pull/112/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=112&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6766
  Stats: 16 lines in 3 files changed: 14 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jmc/pull/112.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/112/head:pull/112

PR: https://git.openjdk.java.net/jmc/pull/112