Initializes PrioritySpec with the |stream_id| of the stream to depend on with |weight| and its exclusive flag. If |exclusive| is true, exclusive flag is set.
Packs the PrioritySpec in |buf|. This function assumes |buf| has enough space for serialization.
Unpacks the priority specification from payload |payload| of length |payload.length| to |pri_spec|. This function assumes the |payload| contains whole priority specification.
The structure to specify stream dependency.