RFR: 6787: Simplified stack trace tree model [v7]

[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 with a new target base due to a merge or a rebase. The pull request now contains 20 commits:

 - 6787: merge latest master and resolve conflicts
 - 6787: fix flame graph inverted stack selection
 - 6787: use 'not available' message when no events match
 - 6787: remove explicit nodeId handling and lookup map
 - 6787: make Node reference children instead of map lookup
 - 6787: add JSON tests for inverted flamegraphs
 - 6787: s/JSON/Json/ in all new files
 - 6787: more fixing of copyright notices
 - 6787: remove alphabetical sorting of nodes, use insertion order
 - 6787: add documentation, fix copyright notices
 - ... and 10 more: https://git.openjdk.java.net/jmc/compare/73fce3fd...3cd7473c

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Changes: https://git.openjdk.java.net/jmc/pull/159/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=159&range=06
  Stats: 1395 lines in 25 files changed: 943 ins; 441 del; 11 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