RFR: 6766: Add heap usage before and after GC to the Garbage Collections table [v2]

[Jean-Philippe Bempel]

> Columns are hidden for now
> 
> Here some screenshots of the changes:
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> ![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)

Jean-Philippe Bempel has updated the pull request incrementally with one additional commit since the last revision:

  fix constant names with underscore

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/112/files
  - new: https://git.openjdk.java.net/jmc/pull/112/files/962b2453..d03089d6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=112&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=112&range=00-01

  Stats: 4 lines in 2 files changed: 0 ins; 0 del; 4 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