[memory] JavaBuilder.oldState leaks into Workspace.tree

In the heapdump of https://github.com/eclipse-platform/eclipse.platform.releng.aggregator/pull/1612 most memory was hold by org.eclipse.core.internal.resources.Workspace.tree.
VisualVM:

JDKMC:

Workspace.tree is meant to store the state of the Workspace. It does so by storing the history of changes (DeltaDataTree) to the workspace. Interestingly the workspace also stores all IncrementalBuilders which again contain references to the old trees.
I.e the Workspace stores a history of things that contain their history.
That org.eclipse.core.internal.events.InternalBuilder.oldState sounds like a leak - or say - something that unintentionally stores more memory then intended.

I have however no idea how to reduce that since an IncrementalBuilders needs to know the oldState to calculate increments. Just that the old state is stored again as a history feels odd.

Use the linked heap-dump analysis as the starting point, then read Workspace.tree, DeltaDataTree, and InternalBuilder.oldState, including JavaBuilder's oldState references. Trace why builder state remains retained and identify the intended lifecycle; the issue is done when a reviewed approach reduces unintended retention and includes evidence that incremental builds still work correctly.

Written by the indexing model from the issue text.