indexer: Allocate new db handle for every index scan

ForestDB db handle is not thread-safe and hence it cannot be shared
among db scan threads. ForestDB iterator alloc helper function
is refactored to create new db handle during iterator creation.

Change-Id: I8fcbde660eae064e81bcd984211e3ea677522c73
Reviewed-on: http://review.couchbase.org/43910
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>