store: Add a functional object cache and use it in 'lower-object'.

store: Add a functional object cache and use it in 'lower-object'.

This leads to ~25% improvements on things like:

  guix system build desktop.tmpl --no-grafts -d

* guix/store.scm (<nix-server>)[object-cache]: New field.
* guix/store.scm (open-connection): Initialize it.
(cache-object-mapping, lookup-cached-object, %mcached): New procedures.
(mcached): New macro.
* guix/gexp.scm (lower-object): Use it.
* guix/grafts.scm (grafting?): New procedure.