perf: reduce the mongodb topology poolsize

apps/emqx_mongodb/src/emqx_mongodb.app.src

@@ -1,6 +1,6 @@
 {application, emqx_mongodb, [
     {description, "EMQX MongoDB Connector"},
-    {vsn, "0.1.7"},
+    {vsn, "0.1.8"},
     {registered, []},
     {applications, [
         kernel,

apps/emqx_mongodb/src/emqx_mongodb.erl

@@ -133,7 +133,8 @@ fields(topology) ->
                 pos_integer(),
                 #{
                     importance => ?IMPORTANCE_HIDDEN,
-                    default => 10
+                    %% In most cases we don't need the topology pool as we use ecpool
+                    default => 1
                 }
             )},
         {max_overflow, fun max_overflow/1},