RFR: 7073: Exceptions in FlameGraph

[Alex Ciminian]

This PR fixes the error reported in the ticket:

Exception in thread "FlameGraphModelCalculation-1" java.lang.ClassCastException: 
class org.openjdk.jmc.flightrecorder.internal.parser.v1.StructTypes$JfrThread cannot be cast
to class org.openjdk.jmc.common.IMCStackTrace

We remove the use of the deprecated `ItemToolkit.accessor` method and resolve the stacktrace accessor independently for every `IItemIterable`.

I saw that `accessor` was using `CachingAccessor` internally. I'm not sure if this change has any performance implications but I assume this is a better way to do it since the old way is deprecated.

The output in the ticket shows that the graph view has a similar problem:

Exception in thread "GraphModelCalculation-2" java.lang.ClassCastException: 
class java.lang.String cannot be cast to class org.openjdk.jmc.common.IMCStackTrace

We do the same change in `StacktraceGraphModel` where we skip over iterables that don't have a stacktrace accessor.

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Commit messages:
 - Make StacktraceGraphModel accept IItemIterables with no stacktrace
 - Make StacktraceTreeModel accept IItemIterables with no stacktrace

Changes: https://git.openjdk.java.net/jmc/pull/187/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jmc&pr=187&range=00
  Issue: https://bugs.openjdk.java.net/browse/JMC-7073
  Stats: 46 lines in 2 files changed: 12 ins; 17 del; 17 mod
  Patch: https://git.openjdk.java.net/jmc/pull/187.diff
  Fetch: git fetch https://git.openjdk.java.net/jmc pull/187/head:pull/187

PR: https://git.openjdk.java.net/jmc/pull/187