#20 add caching for realms

[Max Cooper]

SecurityFilter needs caching for high performance.

The same design that allows realm adapters was also
intended to support caching by decorating a realm with
some kind of cache mechanism. This design allows you to
stack on as many realms as you want, so you can mix and
match adapters, logging, caching, etc. as much as you
want. This also allows application developers to provide
their own realm decorations, but SecurityFilter should come
with some caching support as part of the distribution.

It may be worth allowing a user to specify caching using a
special tag (<realm-cache>?) to make it easier to
configure. The multiple <realm> thing can be confusing.
But it would be nice to avoid changing the DTD since it isn't
technically necessary in this case. And it may just be
*more* confusing if there are two ways to specify the
caching realm. It isn't clear to me at this point which is the
better approach.

-Max