Better support between couchstore to Magma migration

So far our proposed strategy for migration from Couchstore to Magma has been to do XDCR (either self-xdcr or to a different cluster) from a Couchstore bucket to a Magma bucket.
Ian raised an objection to this in the Magma PRD, saying that it is not a great user experience and we should try to do better. In a subsequent discussion in the PRD comment, the following additional migration options are suggested by Perry. We should explore these, some seem difficult of course, but worth discussing:

Option 1:
Use Swap Rebalance during upgrade to move vbuckets from Couchstore to Magma. So rebalance reads Couchstore data from the old node and writes Magma data to the new swapped in node. This of course means that we have to support hybrid buckets – where some vbuckets are Couchstore and some are Magma. Sounds like a big technical hurdle to me, but I would be more than happy to be wrong.
The big advantages of this approach are that it is automatic, online, and does not need 2x the space of the entire dataset.

Option 2:
Explicit offline option to switch a bucket from Couchstore to Magma.

• User changes bucket storage type and takes it offline
• Starts a migration tool which reads from Couchstore vbuckets and writes to Magma vbuckets.
• Online the bucket after the migration

Disadvantage is that it is offline, but can be done vbucket by vbucket without requiring 2x disk space of total data set

Option 3:
Backup a Couchstore bucket, restore it into a Magma bucket. This probably should work already, might be a matter of testing only. But Patrick would know better of course.