Vacuum the package derivations table after each job

As each job deletes lots of rows in the table, vacuum it to free up space for
the next job.

I don't think this is something to do widely, but I wanted to try it out for
the tables which have a lot of churn.

guix-data-service/jobs/load-new-guix-revision.scm

@@ -1758,6 +1758,11 @@ SKIP LOCKED")
               (record-job-succeeded conn id)
               (record-job-event conn id "success")
               (exec-query conn "COMMIT")
+
+              (with-time-logging
+                  "vacuuming package derivations by guix revision range table"
+                (vacuum-package-derivations-table conn))
+
               #t)
             (begin
               (exec-query conn "ROLLBACK")

guix-data-service/model/package-derivation-by-guix-revision-range.scm

@@ -23,6 +23,7 @@
   #:export (delete-guix-revision-package-derivation-entries
             insert-guix-revision-package-derivation-entries
             update-package-derivations-table
+            vacuum-package-derivations-table
             rebuild-package-derivations-table))
 
 (define (delete-guix-revision-package-derivation-entries conn
@@ -149,6 +150,11 @@ LOCK TABLE ONLY package_derivations_by_guix_revision_range
 
   #t)
 
+(define (vacuum-package-derivations-table conn)
+  (exec-query
+   conn
+   "VACUUM package_derivations_by_guix_revision_range"))
+
 (define (rebuild-package-derivations-table conn)
   (with-postgresql-transaction
    conn