Support storing attachments in a separate bucket

[adamcfraser]

Couchbase Server best practices recommend keeping binary and json data in separate buckets. Sync Gateway should support an optional 'binary bucket' for attachment storage.

This will reduce view indexing time, and avoid attachment overhead on Sync Gateway's mutation feed.

Old revision bodies (currently stored as binary) could also be moved to this bucket, although their shorter lifespan makes this less critical.

• [ ] Define a binaryBucket in the config and in the database context
• [ ] Use the binary bucket when writing attachments
• [ ] Use the binary bucket when writing old revision backups
• [ ] Ensure that the Sync Gateway views/design docs aren't being created for the binary bucket

[adamcfraser]

Longer-term we are still evaluating other options for attachment storage (including pluggable storage). This solution doesn't change that strategy - just provides more options in the short term.

[adamcfraser]

To support existing deployments that are migrating to using an attachment bucket, we should support the following:

• [ ] If retrieval of an attachment from the attachment bucket fails, fall back to attempting retrieval from the regular bucket
• [ ] In that scenario, if the attachment is found, Sync Gateway should transfer it from the data bucket to the attachment bucket

This doesn't preclude future development of a one-time attachment migration tool.

[adamcfraser]

Closing based on age, priorization - can be revisited in future if escalated.