Add recipient tracking to the Gnus registry. Reindex it on read if necessary.

Add recipient tracking to the Gnus registry.  Reindex it on read if necessary.

* gnus-registry.el (gnus-registry-fixup-registry): Use it if the
tracked field changes.
(gnus-registry-unfollowed-addresses, gnus-registry-track-extra)
(gnus-registry-action, gnus-registry-spool-action)
(gnus-registry-handle-action)
(gnus-registry--split-fancy-with-parent-internal)
(gnus-registry-split-fancy-with-parent)
(gnus-registry-register-message-ids): Add recipient tracking on spool,
move, and delete actions, and for fancy splitting with parent.
(gnus-registry-extract-addresses)
(gnus-registry-fetch-recipients-fast)
(gnus-registry-fetch-header-fast): Convenience functions.
(gnus-registry-misc-test): ERT test of
`gnus-registry-extract-addresses'.

* registry.el (registry-reindex): New method to recreate the secondary
registry indices.