sstables: consumer: reuse the fragmented_temporary_buffer in read_bytes()
sstables: consumer: reuse the fragmented_temporary_buffer in read_bytes()

read_bytes destroys and creates a vector for every value it reads.
This happens for every cell.
We can save a bit of work by reusing the vector.

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