(perf)db: require optional libraries only once per function (#1944)

Require optional libraries 'org-ref and 'oc only once per function run:
org-roam-db-update-file and org-roam-db-sync will call these requires at
the top level instead of within the link mapper.

org-roam-utils.el

@@ -34,6 +34,11 @@
 
 (require 'org-roam)
 
+(defun org-roam-require (libs)
+  "Require LIBS."
+  (dolist (lib libs)
+    (require lib nil 'noerror)))
+
 ;;; String utilities
 ;; TODO Refactor this.
 (defun org-roam-replace-string (old new s)