Use xref instead of etags

[sktoiva]

xref-push-marker-stack is the up-to-date way of pushing point to marker
stack. This should be used instead of find-tag-marker ring that has been
marked as obsolete in emacs 25.1 (https://github.com/emacs-mirror/emacs/blob/3af9e84ff59811734dcbb5d55e04e1fdb7051e77/lisp/progmodes/etags.el#L184)

Implementation details:

• find-tag-marker-ring was a variable alias for xref--marker-ring
• xref-push-marker-stack is implemented as:

 (defun xref-push-marker-stack (&optional m)
  "Add point M (defaults to `point-marker') to the marker stack."
  (ring-insert xref--marker-ring (or m (point-marker))))

• That's identical with the old code in cider-common (minus the optional m param)

This pull request replaces etags with xref and uses the recommended method, and fixes #3071

[bbatsov]

Thanks!