This method handles each request as it comes in from the
server. It is a hunchentoot handler and has access to all hunchentoot
dynamic variables. The default implementation executes a user
action (if any) and renders the main composite wrapped in HTML
provided by 'render-page'. If the request is an AJAX request, only the
dirty widgets are rendered into a JSON data structure. It also invokes
user supplied 'init-user-session' on the first request that has no
session setup.

'handle-client-request' immediately returns '+http-not-found+' if it
sees a mime type on the script name (it doesn't handle what could be
files because these mess with callback functions and break some
widgets that depend on them).

Additionally, on the first request a session is created and a client
is forced to redirect. At this point if the cookie is sent, session
information is removed from the URL, otherwise the URL is left in
tact. This is done so that session information appears on the URL for
clients that don't support cookies (this way AJAX requests followed by
a refresh will work).

This function also manages lists of callback functions and calls them
at different points before and after request. See 'request-hook'.

Override this method (along with :before and :after specifiers to
customize behavior).