Save instantiated gnus-search engines in an alist

Save instantiated gnus-search engines in an alist

So we aren't re-instantiating (and potentially configuring) them with
every search.

* lisp/gnus/gnus-search.el (gnus-search-engine-instance-alist): New
variable holding server->engine mapping.
(gnus-search-server-to-engine): See if we've already instantiated this
server. If so, return it. If not, instantiate it and save in the above
variable.
(gnus-search-shutdown): Shutdown function clearing the above alist.