indexer: Discover projectors using ClusterInfoCache

Migrated existing kv_sender code to make use of
ClusterInfoCache for discovering projectors across
the cluster.

Change-Id: I9611eb0557c3ff7a673058154c114d59e90c71aa
Reviewed-on: http://review.couchbase.org/43714
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Sarath Lakshman <sarath@couchbase.com>