* (db/snapshot): add more visitors
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): fix lint error
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): add some Iterators
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>

* (db/snapshot): update visitors
Signed-off-by: Npeng.xu <peng.xu@zilliz.com>