Rework loading revision data

These changes were motivated by switching to a mechanism of loading data that
isn't dependent on the big advisory lock that prevents more than one revision
from being processed at a time.

Since INSERT ... RETURNING id; is used, this can block if another transaction
inserts the same data, and then cause an error when that transaction
commits. The solution is to use ON CONFLICT DO NOTHING, but you have to handle
the case when the INSERT doesn't return an id since the other transaction has
inserted it.

This commit rewrites insert-missing-data-and-return-all-ids to do as described
above, as well as being more efficient in how existing data is detected and to
use more vectors. Other utilities for inserting data are added as well.

tests/model-license-set.scm

@@ -9,15 +9,15 @@
 (test-begin "test-model-license-set")
 
 (define license-data
-  '((("License 1"
-      "https://gnu.org/licenses/test-1.html"
-      "https://example.com/why-license-1"))
-    (("License 1"
-      "https://gnu.org/licenses/test-1.html"
-      #f)
-     ("License 2"
-      #f
-      #f))))
+  '#((("License 1"
+       "https://gnu.org/licenses/test-1.html"
+       "https://example.com/why-license-1"))
+     (("License 1"
+       "https://gnu.org/licenses/test-1.html"
+       #f)
+      ("License 2"
+       #f
+       #f))))
 
 (with-postgresql-connection
  "test-model-license-set"