RFR: 6787: Simplified stack trace tree model [v2]

[Alex Ciminian]

> This PR adds a new way to represent JFR data - we aggregate stacktraces into a tree structure which we then allow to be serialised to JSON so we can use it in the `FlameGraphView`.
> 
> The `Node` and `AggregatableFrame` classes were initially copied from the new graph representation then slightly modified. Once the two implementations are merged and stabilised we should consider unifying these models.
> 
> We've validated the output of the new JSON serialisation against the previous flame graph JSON implementation, before removing the legacy code. The intermediary commits have tests that check the output is identical for one of our test recording, using the various configuration options (with/without a target attribute, with thread root at top/bottom).

Alex Ciminian has updated the pull request incrementally with two additional commits since the last revision:

 - 6787: s/JSON/Json/ in all new files
 - 6787: more fixing of copyright notices

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Changes:
  - all: https://git.openjdk.java.net/jmc/pull/159/files
  - new: https://git.openjdk.java.net/jmc/pull/159/files/974a229c..ef24f6a6

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jmc&pr=159&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jmc&pr=159&range=00-01

  Stats: 215 lines in 8 files changed: 99 ins; 96 del; 20 mod
  Patch: https://git.openjdk.java.net/jmc/pull/159.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/159/head:pull/159

PR: https://git.openjdk.java.net/jmc/pull/159