= MDC

The MDC class implements a copy of the Mapped Diagnostic Context, which
is part of the Apache Log4j library.  See the NDC documentation for
more details.  MDCs are much like NDCs, but instead of a stack context
it uses a map for holding this information.

This allows for selection of information out of the map when
the log message is being created.

MDCs are thread safe, and are unique to each thread.

An important difference between MDCs in Log4r vs Log4j is that they
only inherit from the main thread.  Ruby treats all new threads as
being the children of the main thread, even if they are started
from a thread that is not main.
