gnus-registry.el changes

[Ted Zlatanov <tzz@lifelogs.com>]

- tracking by subject: say you get messages with the same subject but
  no references header (yes it's true, some MUAs don't generate those!
  the horror!) - the registry will track them by subject.  There is
  some minimal tuning with gnus-registry-minimum-subject-length and I
  simplify the subject.  I had to modify quite a few backends to pass
  a subject to nnmail-cache-insert.  Let's see how well it works.  My
  tests were encouraging.

- caching extra data: for extra data like the subject or mtime, it's
  all in an alist, and it's slow to traverse that repeatedly.  So I
  cache it dynamically in a hashtable inside the registry.  The
  secondary hashtable does not get saved with the registry.

The changes were pretty major so if gnus-registry.el barfs please
let me know...  I tested the basics.

Thanks
Ted