mx sstable reader: don't yield if the data buffer is not empty
mx sstable reader: don't yield if the data buffer is not empty

The skip() method returns a skip_bytes object if we want to
skip the entire buffer, otherwise it returns a proceed::yes
and trims the buffer.

If the buffer is only trimmed we don't need to interrupt
the coroutine, we simply continue instead.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla