Tweak class caching in node (de)serializers.

[milesziemer]

Prior to this PR, class caching mechanisms in DefaultNodeDeserializers
and DefaultNodeSerializers would cache classes the first time they were
loaded. If the class was loaded again using a new ClassLoader, and then
retrieved from the cache, there would be a mismatch in the loaded class
and the one retrieved from the cache because class equality is dependent
on the ClassLoader used. This caused class cast exceptions in the language
server.

Based on discussion with Michael offline, this PR introduces a modified
caching mechanism that associates a key-value pair with an instance
of a loaded class. When retrieving a value from the cache, the associated
class is also provided, and the cache will be invalidated if the associated
class is not referentially equal to the currently cached class. This modified
caching mechanism replaces caches in DefaultNodeSerializers and
DefaultNodeDeserializers.

Testing

• Test class for the new caching mechanism
• Added test case to ModelAssemblerTest for loading external jars with trait
  implementations multiple times. The jar and model used for this test case
  was taken from the language server.
  By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[milesziemer]

Opening this as a draft because it still needs a test added, but I still have to think about how exactly to test with loading classes in.

[milesziemer]

Looking at this again and trying to update some of the comments, something started to seem not quite right. @mtdowling I was going off your suggestions, but either this implementation isn't exactly what we want, or it needs to be integrated into the node (de)serializers differently. See the diffs DefaultNodeDeserializers:626 and DefaultNodeSerializers:259: the class that is being cached, ObjectMapper and ClassInfo respectively, do not have the same class as the keys of the cache, Class and Class respectively. I added failing test case to show why this doesn't work.

Maybe we can use the previous implementation and this new one together, or we need to be able to use a different class as the key and value in IdentityClassCache.

[kubukoz]

Hey @milesziemer, can you share an estimate of when this might be released? It'd be much appreciated, thanks!