Create new introduction circuit when DosParams changes
When the DosParams extension changes, we should create new circuits to our introduction points with the updated value.
From a comment introduced in !1740 (merged):
// TODO HSS:
//
// We want to make a new introduction circuit if our dos parameters change,
// which means that we should possibly be watching for changes in our
// configuration. Right now, though, we only copy out the configuration
// on startup.