Use new Flusher for different flushes

Flusher stores the indexInstMap and indexPartnMap. Same flusher
cannot be used for different flushes. Create a new instance of
flusher for each flush(its not costly).

Change-Id: I62858b89138e4a77b6bbd72be0fb6aa544643ef3
Reviewed-on: http://review.couchbase.org/41418
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>