CBIDXT-107 add/delete buckets and its engines.

- added method receivers to set corresponding flags.
- added protobuf documentation on API semantics.

Change-Id: I9ca98c661f0d76a7323a80dcfe82de15b634e212
Reviewed-on: http://review.couchbase.org/39260
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>