Don't cache JRuby runtime in static fields

[headius]

There are a few places where we cache the JRuby runtime or runtime-specific objects in a static field, such as here with a WeakReference:

json/java/src/json/ext/RuntimeInfo.java

Lines 83 to 91 in 7d2ad6d

	public static RuntimeInfo forRuntime(Ruby runtime) {
	synchronized (RuntimeInfo.class) {
	if (runtime1.get() == runtime) return info1;
	RuntimeInfo cache = null;
	if (runtimes != null) cache = runtimes.get(runtime);
	assert cache != null : "Runtime given has not initialized JSON::Ext";
	return cache;
	}
	}

The runtimes cached in weak references will not "leak" but it may take longer for the runtime to be collected than normal. In addition, the cost of traversing the weak reference may offset any gains from caching runtime objects.

These should be removed and cached in a runtime-safe way.