Send dedup: Track re-assigned chunks when autopruning

[tasket]

Currently when autopruning occurs during send -d a removed session's chunks will all be marked 'None' indirectly, regardless of whether they were deleted or moved to another session's index. This is done to maintain integrity but can make deduplication during send less effective.

Enhance merge_sessions() to update in-memory dedup index more accurately at the chunk level instead of at the session level.