Make preserve work again (hopefully). It never worked since the

Make preserve work again (hopefully). It never worked since the
transition to DB3.
We don't want db to write-out the underlying re_source, so we
tell it that it's really just a DB_TREE database for the duration of sync.
Hopefully we can convince the people at SleepyCat to reimplement
R_RECNOSYNC, so we can remove this hack.