Fix second argument type on signed atomic add/sub

Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>
Use atomic increment for session sequence

Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>
Sessionid is in the session, don't allocate

Signed-off-by: Adam C. Emerson <aemerson@linuxbox.com>