The data to be sent for this DATA frame.
The flag to indicate whether EOF was reached or not. Initially |eof| is 0. It becomes 1 after all data were read.
The flags of DATA frame. We use separate flags here and http2_data frame. The latter contains flags actually sent to peer. This |flags| may contain END_STREAM and only when |eof| becomes nonzero, flags in http2_data has END_STREAM set.
The flag to indicate that DataFlags.NO_COPY was used.
struct used for DATA frame