Config changes

- a source's interval must be greater than 0: a source is turned off with
  enabled, not with a zero interval, so JSCSourcesSource rejects an interval of
  0 or less (its validate is promoted to each source's section);
- max_message_size rejects -1: JSCServerWebSocket rejects a max_message_size
  of -1 (util.ByteSizeNoLimit), as a client could then exhaust the server's
  memory;
- every setting documents what 0 or empty means (zero/none, no limit or a
  default).

internal/config/conf_server_timeouts.go

@@ -9,7 +9,8 @@ import (
 const JSCKServerTimeouts string = "timeouts"
 
 // JSCServerTimeouts configures the server's timeouts. ShutdownTimeout is how
-// long shutdown waits for clients to close.
+// long shutdown waits for clients to close; 0 or less means it waits until
+// they have.
 type JSCServerTimeouts struct {
 	ShutdownTimeout time.Duration `cfg:"shutdown" default:"15s"`
 }