How to tell jenkins not to kill processes after successful execution of job in multijob project?

Jenkins uses a special mechanism to cleanup child processes. It looks for process with BUILD_ID matching the build number. You can set that to different value and jenkins will skip killing the process.

From the docs:

https://wiki.jenkins-ci.org/display/JENKINS/ProcessTreeKiller

BUILD_ID=dontKillMe /your/mongodb/process

or for pipelines:

JENKINS_NODE_COOKIE=dontKillMe /your/mongodb/process