Optimize xapi<->xenopsd communication (#6549)

Some refactoring, but the most important commit is this one-line change
that greatly improves things on its own:

---

xenopsd: Remove `data/updated` from the list of watched paths

"data/updated" is not read or used anywhere in xenopsd or xapi:
* xapi_guest_agent's last_updated field is just Unix.gettimeofday ().
* xapi_xenops removes "data/updated" from the guest agent state
altogether before
checking if it's changed:

```
let ignored_keys = ["data/meminfo_free"; "data/updated"; "data/update_cnt"]
```

So there is no need to watch this path at all.

This greatly reduces unnecessary traffic between xapi and xenopsd, since
any VM
with a guest agent would write to data/updated once every 60 seconds,
which
would generate a Dynamic.Vm event, making xapi call xenopsd's VM.stat to
rescan
the domain's xenstore tree and perform several hypercalls. Almost
always, this
would be completely unnecessary as nothing else about the VM would
change, but
a lot of work would be done anyhow.

Author: last-genius

ocaml/xapi-idl/lib/updates.ml

@@ -66,6 +66,7 @@ functor
           )
 
         let inject_barrier id filterfn t =
+          let filterfn key _ = filterfn key in
           ( {
               map= t.map
             ; barriers=

ocaml/xapi-idl/lib/updates.mli

@@ -64,7 +64,7 @@ module Updates : functor (Interface : INTERFACE) -> sig
   (* [inject_barrier n p t] Inject a barrier identified by [n] into [t]. The
      barrier will contain a snapshot of all current updates that match the
      predicate [p]. *)
-  val inject_barrier : int -> (Interface.Dynamic.id -> int -> bool) -> t -> unit
+  val inject_barrier : int -> (Interface.Dynamic.id -> bool) -> t -> unit
 
   (* Removes a barrier *)
   val remove_barrier : int -> t -> unit

ocaml/xapi-idl/lib_test/updates_test.ml

@@ -84,7 +84,7 @@ let test_inject_barrier () =
   let u = M.empty scheduler in
   M.add update_a u ;
   M.add update_b u ;
-  M.inject_barrier 1 (fun _ _ -> true) u ;
+  M.inject_barrier 1 (fun _ -> true) u ;
   M.add update_a u ;
   M.add update_c u ;
   let barriers, updates, _id = M.get "dbg" None (Some 1) u in
@@ -107,7 +107,7 @@ let test_remove_barrier () =
   let u = M.empty scheduler in
   M.add update_a u ;
   M.add update_b u ;
-  M.inject_barrier 1 (fun _ _ -> true) u ;
+  M.inject_barrier 1 (fun _ -> true) u ;
   M.add update_a u ;
   M.add update_c u ;
   M.remove_barrier 1 u ;
@@ -125,7 +125,7 @@ let test_inject_barrier_rpc () =
   let u = M.empty scheduler in
   M.add update_a u ;
   M.add update_b u ;
-  M.inject_barrier 1 (fun _ _ -> true) u ;
+  M.inject_barrier 1 (fun _ -> true) u ;
   M.add update_a u ;
   M.add update_c u ;
   let barriers, updates, _id = M.get "dbg" None (Some 1) u in
@@ -175,7 +175,7 @@ let test_filter () =
 let test_dump () =
   let u = M.empty scheduler in
   M.add update_a u ;
-  M.inject_barrier 1 (fun _ _ -> true) u ;
+  M.inject_barrier 1 (fun _ -> true) u ;
   let dump = M.Dump.make u in
   let dumped_rpc = M.Dump.rpc_of_dump dump in
   let expected_rpc =