grafts: Cache the derivation/graft mapping for the whole session.

grafts: Cache the derivation/graft mapping for the whole session.

Partly fixes <https://bugs.gnu.org/41702>.
Reported by Lars-Dominik Braun <ldb@leibniz-psychology.org>.

Previously, 'graft-derivation' would start anew at every call.  When
creating a profile with lots of packages, it would potentially do the
same work multiple times.  The per-session cache addresses this.  It
increases the derivation-graft-cache hit rate from 77.9% to 80.1% on:

  GUIX_PROFILING="derivation-graft-cache" ./pre-inst-env \
    guix environment --ad-hoc libreoffice inkscape krita darktable -n

The effect is more visible on the pathological case below, where cache
hit rate goes from 75% to 87% and wall-clock time from 5.0s to 3.5s:

  GUIX_PROFILING="derivation-graft-cache" ./pre-inst-env \
    guix environment --ad-hoc r-learnr --search-paths

* guix/grafts.scm (%graft-cache): New variable.
(graft-derivation): Add calls to 'store-connection-cache' and
'set-store-connection-cache!'.