MB-7413 Count # of seqs processed for initial build

The new seqs processed stat was only being computed for
incremental index updates. Account it for the initial
index build as well.

Change-Id: I9c71daec96cdb42b7dad76c9cacf068be6d60487
Reviewed-on: http://review.couchbase.org/24125
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>