database: register-items: reduce transaction scope.

It was made transactional in a4678c6ba18d8dbd79d931f80426eebf61be7ebe, with
the reasoning to prevent broken intermediate states from being visible. I
think this means something like an entry being in ValidPaths, but the Refs not
being inserted.

Using a transaction for this makes sense, but I think using one single
transaction for the whole register-items call is unnecessary to avoid broken
states from being visible, and could block other writes to the store database
while register-items is running. Because the deduplication and resetting
timestamps happens within the transaction as well, even though these things
don't involve the database, writes to the database will still be blocked while
this is happening.

To reduce the potential for register-items to block other writers to the
database for extended periods, this commit moves the transaction to just wrap
the call to sqlite-register. This is the one place where writes occur, so that
should prevent the broken intermediate states issue above. The one difference
this will make is some of the registered items will be visible to other
connections while others may be still being added. I think this is OK, as it's
equivalent to just registering different items.

* guix/store/database.scm (register-items): Reduce transaction scope.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

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