Fix cyclic import dependency issue

move partition_instance.go and slice_container.go to indexer

Change-Id: I93e2edce9181b8bccbc9135a56f56ba6e448a602
Reviewed-on: http://review.couchbase.org/38490
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>