Cleaner thread too slow

[pmconrad]

1. Version of JNA and related jars. 5.13.0
2. Version and vendor of the java virtual machine: openjdk-17.0.8.0 and others
3. Operating system: Linux (Debian, OpenSUSE)
4. System architecture (CPU type, bitness of the JVM), x86_64
5. Complete description of the problem:
   I have an application that will sporadically run into a situation where multiple threads are creating lots of short-lived jna.Structure and jna.Union objects. The problem is that jna's single Cleaner thread cannot keep up freeing objects created by multiple parallel threads, so eventually all heap memory will be used up despite the fact that it could be GC'ed.
6. Steps to reproduce:
   See test case.