Fix memory leaks

[unp1]

Fixes two memory leaks:

• GUI: nodes of closed proofs were still kept in memory due to references from the GUIProofTreeModel and possibly sequent views
• Thread-Local Caches: two thread-local caches kept also references; these would have been eventually replaced, but as they are kept by the Java default Swing threads (assume N threads) it could happen that up to N proofs remained if each next proof would have been single core and execute don a different one

Encountered when working on a proofs for a paper on ETAPS and the GUI showed 3GB used with all proofs removed (even after running GC). Now after running GC only 100 MB usage remains.

Intended Change

Fix memory leak

Type of pull request

• Bug fix (non-breaking change which fixes an issue)

Ensuring quality

• I have tested the feature as follows:
  • checked memory usage in the GUI after loading, running a proof (Which used 6GB) and closing it (several times); before fix: 3GB remained; after fix: only 100 MB
• I have checked that runtime performance has not deteriorated.
  • used the benchmarking tool

Additional information and contact(s)

The contributions within this pull request are licensed under GPLv2, or (at your option) any later version, for inclusion in KeY.