bug#43356: fix misguided

[Madhu]

Please consider reverting the reverts and maintaining the old code
paths.  Old functionality is *not* available after the change commit
def34a20.

The restored "old behaiour" which is desired includes that

(a) *all* articles which are read when
(b) gnus-agent is t and
(c) and the group is agentised
are cached in the agent cache.

This means that 1) articles in groups which are not agentised should
not be cached in the agent cache. 2) when gnus-agent is nil articles
should not be cached in the agent cache.

The commit def34a20 suggests that one could get the old behaviour by
adding `gnus-agent-fetch-selected-article' to
`gnus-select-article-hook'

First problem is that this adds a 0.5 to 2 seconds overhead on
selecting each article.  Initial tests with ELP show that
`gnus-agent-fetch-articles' which is called for each article is over
100 times slower than than `gnus-agent-store-article' (which this
commit deprecates and removes from use).

When articles are missed by the hook there is no user interface to
ensure that the articles can be cached in the agent cache. There is no
way to fetch already fetched articles to the agent cache except by
selecting them again (painfully slow)

whether or not bug#8502 was an "erroneous feature request" this
particular "fix" is erroneous in the functionality it removes. here is
my rant on emacs-help:
https://lists.gnu.org/archive/html/help-gnu-emacs/2020-10/msg00037.html