Optimize sorter view record overhead memory footprint

The view record structure used by sorter has an overhead
of 40 bytes per record which accounts for large part of
memory allocated for sorting. Optimized the structure to
have only 7 bytes per record overhead.

The heap profiling results show that for 64MB file of records,
memory footprint has reduced from 140 MB to 86 MB.

Change-Id: I479699c93c3bc221c8d051bf9af840e76045e3fc
Reviewed-on: http://review.couchbase.org/33037
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>