ThreadContext.clear() doesn't tidy up MDC values properly.

[tallpsmith]

From [email protected] on March 10, 2011 16:48:57

ThreadContext.clear()

doesn't actually do the inner MDC.clear()
that's because MDC doesn't HAVE a clear..
but it's not clearing...
we really need to iterate over the MDC keys and remove.
because otherwise the MDC becomes tainted.

Original issue: http://code.google.com/p/parfait/issues/detail?id=11