add 'naturl_key' to blob meta
add 'naturl_key' to blob meta

This is needed when dumping blob meta to prevent primary key
collisions when re-loading the data. This is safe since
BlobMeta is not referenced by anywhere by it's primary key

dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq,dimagi/commcare-hq