SearchIdx: Decrement regen_down even for added messages that are later deleted.
Decrement regen_down when visiting messages that appear in %D that we
know will later be deleted. This ensures consistent message numbers are
generated no matter which commit number is on top. Allowing deletes to
propagage separately from the messages they delete without causing
problems.
The v2 trees already do this and when the indexes are deleted and
rebuilt they maintain they commit numbers.
Add a v1 version of the v2reindex test to verify that reindexing is
working properly on v1 as well as v2.
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>