Don't send the subbuffer padding for streaming

For network streaming, with the mmap() mechanism only for now, the
consumer does NOT send the padding over the network. Instead, the size
of the padding is specified in the data header or metadata payload.

The lttng-relayd now is the one appending the zeros to the trace files.

Again, this feature is NOT available yet for splice output.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>