Webapp improvements

[fvilla]

Have the webapp object provide user model and session using abstract virtuals or @webapplication parameters.

Provide callbacks for notification of session creation and deletion.

Enable quick status report on application with http status .

Enable webapp-specific logging (also visualize logs with http log [app]).

Cleanup of web space (session scratch areas) both at session deletion (according to config) and on-demand using a command (http cleanup [app]).