RFR: 6542: Improve calls to render in the flame view

[Miroslav Wengner]

stable flameview generation

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Commit messages:
 - 6542: naming correction, models container check
 - 6542: stacktrace model utils
 - 6542: model assignment fix
 - 6542: cleanup
 - 6542: clean up
 - 6542: partial commit, fixed flameview

Changes: https://git.openjdk.java.net/jmc/pull/78/files
 Webrev: https://webrevs.openjdk.java.net/jmc/78/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JMC-6542
  Stats: 145 lines in 3 files changed: 84 ins; 16 del; 45 mod
  Patch: https://git.openjdk.java.net/jmc/pull/78.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/78/head:pull/78

PR: https://git.openjdk.java.net/jmc/pull/78