Cover functional test for the ticket : CBL-2826

Have a functional test on CBL
1. Create a doc with a blob and save into a db on cbl
2. Have push_pull replication to SGW with continuous true
3. update the same doc on cbl
Get the same doc from db and update the doc with a key and string value (without changing blob).
4. Wait for the updated doc get replicated to sgw
5. Verify there is _attachment key on the SGW and make sure _attachment shows up on both SGW and CBL.