effective config: specific prefetch/trigger case

effective config: specific prefetch/trigger case

The prefetch statement can be overriden by the user, but the user might
specify the prefetch without the trigger value, which needs to be
pulled from the default configuration. Handle this case by directly
getting the default value if needed from the default configuration when
building the effective configuration tree.

Also take care of keeping the values inside their bounds, and simplify
the server configuration code which then just have to read effective
configuration values.