CA-91930: Remove union of subscriptions between event.from invocations.

This fixes an issue whereby when parallel event.from calls were being
processed, each call would get the union of events meant for each. This
causes particular problems when XenCenter is connected as it registers
for _all_ events. Therefore any other calls made to event.from will
receive _all_ events whenever the GUI is connected. Additionally, as
there is no way to generate a token for 'now', this means that the
initial call to event.from will dump the _entire_ database, including
all the messages, requiring reading these from the disk.

Signed-off-by: Jon Ludlam <jonathan.ludlam@eu.citrix.com>

ocaml/xapi/xapi_event.ml

@@ -324,15 +324,14 @@ let from ~__context ~classes ~token ~timeout =
 		objs
 	in
 
-	let all_subs = Mutex.execute sub.m (fun () -> Hashtbl.fold (fun _ s acc -> s.subs @ acc) subscriptions []) in
-	let tables = List.filter (fun table -> table_matches all_subs table) all_event_tables in
+	let tables = List.filter (fun table -> table_matches sub.subs table) all_event_tables in
 
 	let events_lost = ref [] in
 
 	let grab_range t =
 		let tableset = Db_cache_types.Database.tableset (Db_ref.get_database t) in
 		let (msg_gen,messages) =
-			if table_matches all_subs "message" then (!message_get_since_for_events) ~__context sub.last_msg_gen else (0L, []) in
+			if table_matches sub.subs "message" then (!message_get_since_for_events) ~__context sub.last_msg_gen else (0L, []) in
 		(msg_gen, messages, tableset, List.fold_left
 			(fun acc table ->
 				 Db_cache_types.Table.fold_over_recent sub.last_generation