How to save all session in case of hybris restart ?

I know How tomcat stores session by using Persistent manager.

How can i configure Persistent manager for particular/all extension in hybris .

1- I know , It can be done by changing in platform/tomcat/conf/server.xml

2- Which property i need to specify in local.properties file.

3- what changes must made in hybris/conf/tomcat/server.xml file?

This file platform/tomcat/conf/server.xml lost changes after ant all so need permanent solution by following choice 2 or 3. Please suggest if anyone knows better solution.