streaming: Log number of bytes sent and recevied when stream_plan completes
streaming: Log number of bytes sent and recevied when stream_plan completes

It is useful for test code to verify number of bytes sent/received.

It looks like below in the log.

/tmp/out1:INFO  [shard 0] stream_session - \
[Stream #1f3e23f0-db9e-11e5-9cfb-000000000000] bytes_sent = 0, bytes_received = 15760704

/tmp/out2:INFO  [shard 0] stream_session - \
[Stream #1f3e23f0-db9e-11e5-9cfb-000000000000] bytes_sent = 0, bytes_received = 18203964

/tmp/out3:INFO  [shard 0] stream_session - \
[Stream #1f3e23f0-db9e-11e5-9cfb-000000000000] bytes_sent = 33964668, bytes_received = 0

duarten/scylla,duarten/scylla,scylladb/scylla,raphaelsc/scylla,avikivity/scylla,raphaelsc/scylla,scylladb/scylla,avikivity/scylla,kjniemi/scylla,scylladb/scylla,kjniemi/scylla,scylladb/scylla,avikivity/scylla,duarten/scylla,kjniemi/scylla,raphaelsc/scylla