avoid sub-classing ThreadLocal since it will leak in envs such as .wars

e.g. on Tomcat a Warbled up with this change (tested 1.7.13) allows to avoid the following warning : 
```
Jun 27, 2014 4:00:45 PM org.apache.catalina.loader.WebappClassLoader checkThreadLocalMapForLeaks
SEVERE: The web application [/hellowarld] created a ThreadLocal with key of type [org.joni.StackMachine$1] (value [org.joni.StackMachine$1@763c3222]) and a value of type [java.lang.ref.WeakReference] (value [java.lang.ref.WeakReference@69ff92d2]) but failed to remove it when the web application was stopped. Threads are going to be renewed over time to try and avoid a probable memory leak.
```
... and correctly release memory

this issue is related to https://github.com/jruby/jruby/pull/1772