CBIDXT-28: Added `Single` PartitionScheme.

Mostly this is how endpoint resolution will look like in sherlock.
- partn_single.proto, contains message structure for single-partition-scheme.
- partn_single.go, contains logic to resolve endpoints.
test-partition refactored.
- partn_tp.proto, contains message structure for test-partition-scheme.
- partn_tp.go, contains logic to resolve endpoints.

Change-Id: I5bc82946e49cb193560cf7d822f4ed037920c695
Reviewed-on: http://review.couchbase.org/43241
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>