Replicated Eventing : Notify apps on the replication status of individual documents

PRD: https://docs.google.com/document/d/1fKawqYLoLW5feaChnRzgzpsntOFNW1rp8W9KEdFujms/edit

Priority Reference :https://docs.google.com/spreadsheets/d/1K5EIWoiZifJt1ABygmj_8ppT-6_xS7ojhjxLEOgpHsU/edit#gid=0

The ability for apps to be notified on the status of a replication would be useful for apps to take appropriate actions such as

• Purge documents after successful push replication (https://forums.couchbase.com/t/cb-lite-2-0-purge-after-successful-push-replication/14790) 
• Display individual document sync status which is very useful to prevent users from inadvertently turning off app/tablet in middle of a document sync - when they are only interested in specific documents https://forums.couchbase.com/t/need-event-driven-notification-of-specific-documents-that-are-pushed/16355
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