The Gnus registry keeps track of all received messages in a series of
databases, and allows the user to split follow-up messages into the same
group as the parent, as well as associate

It currently creates database entries for all messages, which slows down
summary buffer creation, as well as Gnus' saving process.

This patch introduces a new customization option,
`gnus-registry-register-all-p', which, when nil, won't create database
entries automatically. The registry will only contain entries that the
user (or other packages) has created manually. If the user isn't using
automatic splitting, this can provide a significant speedup for general
Gnus usage.

Discussions on gnus.general lead me to believe that universal splitting
isn't used all that often, and that most users are very surprised to
find that they have a 50MB registry file on disk. Therefore I've
defaulted this option to nil, which is a change from previous behavior.
It would be perfectly easy to default to t if this seems inappropriate.

Eric