extract DbConn from PlumeRocket #805

in the following Pull Request, we'lle extract DbConn from PlumeRocket, as suggested in #797

use rocket_contrib's #[database] to implement our DbConn
we do not use #[database]'s connection setup, retaining our own init_pool() and ConnectionCustomization!
pass DbConn in as ref instead of Copying it around.

pass DbConn in as ref instead of Copying it around.

This will help with the async migration, as we're currently passing all of PlumeRocket around as ref, which means that it ends up sitting somewhere in a function, until it's .awaited.

This can create lead us into a big mess, which may be impossible to untangle, so this should help not running into that to begin with.

closing in favour of #813

         &self.0
     }
 }
 #[database("plume")]