Hi,
2 options:
1) Increase shutdown time before "escalating to sigterm":
http://answers.ros.org/question/11353/how-to-delay-escalation-to-sig-term-after-sending-sig-int-to-roslaunch/2) You can call /rtabmap/backup service, which will close rtabmap.db, copy to "rtabmap.db.back" then reload rtabmap.db.
$rosservice call /rtabmap/backup
cheers,
Mathieu