When the first time the keep alive timer fires, the _last_stream_bytes
btyes will be zero since it is the first time we update it. The keep
alive timer will be rearmed and fired again. The second time, we find
there is no progress, we close the session. The total idle time will be
2 * keep alive timer.
To make the idle time to close the session be more precise, we reduce
the interval to check the progess and close the session by checking last
time the progress is made.
Message-Id: <c959cffce0cc738a3d73caaf71d2adb709d46863.1456831616.git.asias@scylladb.com>