Bootstrap lookup fix

[felixbarny]

What does this PR do?

Fixes #1450

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix

I've tried creating a test for this (see 9dcaff3) but gave up.

The problem is that the instrumented class will be defined by the system class loader which is also a parent of the IndyPluginClassLoader as it’s the agent class loader in unit tests. Thus, the IndyPluginClassLoader will load the instrumented class from the system class loader before trying to load from the custom class loader. That’s because we want to give precedence to load classes in the agent class loader.
This results in a linkage error as the class resolved by the IndyPluginClassLoader is not the same as the one from the custom class loader whose parent is the bootstrap class loader.
The test also requires that the signature of the advice contains a class that's defined in two different class loader hierarchies. But that class must not be within the package co.elastic.apm.* as these are not available in production in the context of the instrumented methods.

[ghost]

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Build stats

• Build Cause: [Pull request #1458 updated]

• Start Time: 2020-10-27T14:10:02.275+0000

• Duration: 45 min 15 sec

Test stats 🧪

Test	Results
Failed	0
Passed	1615
Skipped	12
Total	1627

[eyalkoren]

Elegant!
Only comment is to try and make sure the new LookupExposer is only loaded once within each class loading hierarchy, for good measures 🙂