Mobile Attachment Cleanup

As part of this enhancement in Lithium, a built-in feature for attachment cleanup will be added to Sync Gateway while retaining the existing shared references to attachments across documents.

PRD: https://docs.google.com/document/d/1sAO14XkBVSd3g4Ie_fpLj-PNOCUXwDcqBjaw-1RUwo8/edit

Won't document this as a new feature because technically it's a bug and technical debt.

Will update the API description for 3.0 as it offers an option to trigger manual compaction of attachments and include behaviour change in release notes