In the new API the biggest change is to implement the only
data_sink_impl::put(span<temporary_buffer>) overload.
Encrypted file impl and sstables compress sink use fallback_put() helper
that generates a chain of continuations each holding a buffer.
The counting_data_sink in transport had mostly been patched to correct
implementation by the previous patch, the change here is to replace
vector argument with span one.
Most other sinks just re-implement their put(vector<temporary_buffer>)
overload by iterating over span and non-preemptively grabbing buffers
from it.
Signed-off-by: Pavel Emelyanov <xemul@scylladb.com>