Use latest structs for Slice/Topology in Flusher

- Use IndexInstMap, IndexPartnMap in Flusher
- Added actual persistence logic using Slice interface
- Improved Flusher interface method names

Change-Id: I93dbf2a66aaf7f6b41739b3d0197525ce4d95814
Reviewed-on: http://review.couchbase.org/38307
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>