The problem is that on the follower side, we set up _session_info too
late, after received PREPARE_DONE_MESSAGE message. The initiator can
send STREAM_MUTATION before sending PREPARE_DONE_MESSAGE message.
To fix, we set up _session_info after we received the prepare_message on
both initiator and follower.
Fixes#869
scylla: streaming/session_info.cc:44: void
streaming::session_info::update_progress(streaming::progress_info):
Assertion `peer == new_progress.peer' failed.
Message-Id: <6d945ba1e8c4fc0949c3f0a72800c9448ba27761.1454476876.git.asias@scylladb.com>