nnima-split-with-parent

[Ted Zlatanov <tzz@lifelogs.com>]

I want nnimap-split-with-parent! :) 

I was looking at the nnmail-split-with-parent code, and I see that the
nnmail cache is used for duplicate detection *and* for splitting with
the parent.  So it's not easy to abstract the nnmail cache as it is to
arbitrary newsgroup names, we might get erroneous warnings about
duplicates.  I think the format for the "new" article-ID cache should
be

backend-name\tarticle-ID\tgroup-name

We can keep the current nnmail cache as well, but this format can
replace the old cache format with very little work.  Maybe we can copy
and fix up the new-cache from the old nnmail-cache if we're upgrading,
and otherwise always use the new-cache file?

We should separate the functionality into gnus-article-cache.el if
it's going to be generalized (or whatever the file is called).

Functions affected (new names are not necessarily final):

nnmail-cache-insert: needs to be generalized to
gnus-article-cache-insert, maybe figuring out the backend name instead
of having it passed in for minimum code changes?

nnmail-cache-fetch-group: needs to be generalized to
gnus-article-cache-fetch-group, and the logic altered a little to
allow for the backend name at beggining of line.  Like the insert
function, maybe it should figure out the current backend name instead
of having it passed in for minimum code changes?

nnmail-cache-id-exists-p: generalize to
gnus-article-cache-id-exists-p, unchanged otherwise

the other nnmail-cache-* functions should be easily migrated to
gnus-article-cache-*

I hope someone is interested in doing this or at least giving me some
feedback...

Ted