Function replicate 

pub async fn replicate(
    source: &dyn Adapter,
    target: &dyn Adapter,
    opts: ReplicationOptions,
) -> Result<ReplicationResult>

Run a one-shot replication from source to target.

Implements the CouchDB replication protocol:

1. Read checkpoint
2. Fetch changes from source
3. Compute revs_diff against target
4. Fetch missing docs from source
5. Write to target
6. Save checkpoint