Implement class unloading

JikesRVM should unload classes that are no longer in use. I finally have a benchmark that needs it

One of the candidate benchmarks for the next release of dacapo is Apache Derby. When running the PseudoJDBCBench benchmark with Derby, dynamic SQL statements such as

Stmt.executeUpdate("UPDATE tellers SET Tbalance = Tbalance + " + delta + " WHERE Tid = " + tid);

result in on-the-fly creation of classes, eventually resulting in the JTOC filling up and the world ending. Running hotspot with the same benchmark and -verbose:class, we see this kind of thing

[Loaded org.apache.derby.exe.acc87100b8x0115xea15x7ebex00000026a740c4 from org.apache.derby.impl.services.reflect.ReflectLoaderJava2]
[Loaded org.apache.derby.exe.acc87100b8x0115xea15x7ebex00000026a740c5 from org.apache.derby.impl.services.reflect.ReflectLoaderJava2]
[Unloading class org.apache.derby.exe.ac6dd780d4x0115xea15x7ebex00000026a74043]
[Unloading class org.apache.derby.exe.ac1d6e40d2x0115xea15x7ebex00000026a74061]

The problem can be reproduced by
svn co https://dacapobench.svn.sf.net/svnroot/dacapobench/benchmarks/trunk dacapo
cd dacapo
ant derby.source
ant derby jar.quick
rvm -jar dacapo.jar derby