Experimenting with per-user access.

Experimenting with per-user access.

        * src/main/java/org/glom/web/server/ConfiguredDocument.java:
Remove the connection, adding it as a parameter to public methods.
Moved setUsernameAndPassword() to
* src/main/java/org/glom/web/server/SqlUtils.java:
as tryUsernameAndPassword().
        * src/main/java/org/glom/web/server/ConfiguredDocumentSet.java:
        Adapted.
        * src/main/java/org/glom/web/server/OnlineGlomServiceImpl.java:
checkAuthentication(): Store a cookie if the authentication was
successful. Also create the database connection and store that too,
for later use.
        * src/main/java/org/glom/web/server/OnlineGlomImages.java:
 * src/main/java/org/glom/web/server/OnlineGlomServlet.java: Share
a base class, which gets the existing connection based on
the cookie.
        * src/main/java/org/glom/web/server/SessionListener.java:
Used to invalidate the connection when the browser session
ends. However, this probably needs to be declared in our web.xml.
        * src/main/java/org/glom/web/server/UserStore.java:
An in-memory store of the remembered credentials (and connections).
This is just a test: It never discards old unused credentials
and loses everything when the server stops.
        * src/test/java/org/glom/web/server/ConfiguredDocumentTest.java:
        * src/test/java/org/glom/web/server/SelfHostConfiguredDocumentTest.java:
Adapted.