Revert "Guard MessagePack with a ThreadLocal where necessary"

This reverts commit d4405a9cce22742a7053b3b5cb45a260a0efb93f.

Unfortunately the ThreadLocal did not solve the problem. As it turns out
the reason that our usage failed is the quite rapid creation of MessagePack
instances in the first place and the reuse of memory after GC collection of
those instances which produce the same hash code as per Object#hashCode();
also see http://bugs.java.com/bugdatabase/view_bug.do?bug_id=6321873
Use a singleton MessagePack instance in all threads