refactor(playout): boot_timestamp can be static

The previous use of interactive was used to hack the boot_timestamp after a settings change/liquidsoap restart.

playout/libretime_playout/liquidsoap/1.4/ls_lib.liq

@@ -98,11 +98,11 @@ end
 def output_to(output_type, type, bitrate, host, port, pass, mount_point, url, description, genre, user, s, stream, name, channels) =
     source = ref s
     def on_error(msg)
-        gateway("stream '#{stream}' '#{boot_timestamp()}' --error='#{msg}'")
+        gateway("stream '#{stream}' '#{boot_timestamp}' --error='#{msg}'")
         5.
     end
     def on_connect()
-        gateway("stream '#{stream}' '#{boot_timestamp()}'")
+        gateway("stream '#{stream}' '#{boot_timestamp}'")
     end
 
     stereo = (channels == "stereo")

playout/libretime_playout/liquidsoap/1.4/ls_script.liq

@@ -1,4 +1,4 @@
-boot_timestamp = interactive.string("boot_timestamp", string_of(gettimeofday()))
+boot_timestamp = string_of(gettimeofday())
 
 web_stream_enabled = ref false
 web_stream_id = ref '-1'