Static ThreadLocals must be weak or soft or not static

While fixing JRUBY-6522, I found that there are a few additional places where we have static ThreadLocals that can cause ClassLoader leaks in environments where a pool of threads may encounter multiple JRuby runtimes, such as when applications are redeployed to a webapp server.

I audited for static threadlocals, and found the following:

org.jruby.ext.ffi.AutoPointer:25

private static final ThreadLocal<ReaperGroup> currentReaper = new ThreadLocal<ReaperGroup>();

org.jruby.ext.ffi.jffi.AllocatedNativeMemoryIO:14

private static final ThreadLocal<AllocationGroup> currentBucket = new ThreadLocal<AllocationGroup>();

org.jruby.ext.ffi.jffi.TransientNativeMemoryIO:43

private static final ThreadLocal<Magazine> currentMagazine = new ThreadLocal<Magazine>();

org.jruby.interpreter.IRBreakJump:13

   private static ThreadLocal<IRBreakJump> threadLocalBJ = new ThreadLocal<IRBreakJump>() {
       public IRBreakJump initialValue() { return new IRBreakJump(); }
    };

org.jruby.interpreter.IRReturnJump:11

    private static ThreadLocal<IRReturnJump> threadLocalRJ = new ThreadLocal<IRReturnJump>() {
       public IRReturnJump initialValue() { return new IRReturnJump(); }
    };

There's also this in org.jruby.Ruby, but I'm not sure if it's a risk; it's explicitly used for thread-local stuff, but via an API we don't publish or recommend. Perhaps it should be removed?

private static ThreadLocal<Ruby> threadLocalRuntime = new ThreadLocal<Ruby>();