JJ: Description from the destination commit:

+ wip exchange_sig

JJ: Description from source commit:
+ signkeys

src/config.ml

@@ -20,9 +20,13 @@ module Exchange = struct
   let port = get "port" |> int
   let bind_to = get "bind_to"
   let master_public_key = get "master_public_key" |> ed25519
+
+  (* TODO Defaults to 0.0 if not specified. *)
   let stefan_abs = get "stefan_abs" |> amount
   let stefan_log = get "stefan_log" |> amount
-  let stefan_lin = get_opt "stefan_lin" |> Option.map float
+
+  let stefan_lin =
+    get_opt "stefan_lin" |> Option.map float |> Option.value ~default:0.0
 
   let aggregator_idle_sleep_interval =
     get "aggregator_idle_sleep_interval" |> duration