Simplify Eventgen (#6188)

This is a small effort to simplify the code within the `Eventgen`
module. The previous style is very unwieldy and makes the file appear
more complicated on the surface than it really is.

---

Passes BVT+BST (209475) but would appreciate review comments as I've
dropped the potential for stdout logging in some places (easy to add
back) and added commentary that may not be fully accurate.

ocaml/idl/datamodel.ml

@@ -10448,7 +10448,18 @@ let all_system =
  * updated must come first. The second field will be automatically * kept
  * up-to-date. *)
 
-(** These are the pairs of (object, field) which are bound together in the database schema *)
+(**
+   These are the pairs of (object, field) which are bound together in
+   the database schema.
+   
+   It is assumed that, for any entry (p, p'), neither p nor p'
+   appears in any other entry. It may be the case that p = p', which
+   is the only instance where some object-field pair may appear more
+   than once.
+   
+   This is implicitly assumed by other code which treats this list -
+   and its symmetric closure - as an association list
+   without duplicate keys. *)
 let all_relations =
   [
     (* snapshots *)

ocaml/idl/ocaml_backend/gen_db_actions.ml

@@ -421,8 +421,7 @@ let db_action api : O.Module.t =
     O.Let.make ~name:"_" ~params:[] ~ty:"unit"
       ~body:
         [
-          Printf.sprintf "Hashtbl.add Eventgen.get_record_table \"%s\""
-            obj.DT.name
+          Printf.sprintf "Eventgen.set_get_record \"%s\"" obj.DT.name
         ; Printf.sprintf
             "(fun ~__context ~self -> (fun () -> API.rpc_of_%s_t \
              (%s.get_record ~__context ~self:(Ref.of_%sstring self))))"