Migrate tracking of Logged-In Users into core.

Hallo Community. I am working on this ticket Log in - OpenMRS Issues

I transferred the CurrentUsers.Java code to the openmrs-core as CurrentUsersService.java class. In addition there is a CurrentUsers.Java Class that extends the CurrentUsersService class. I have also transferred the SessionListener class to the openmrs-core and added a sessionManager class.

I would love a code review. PR: TRUNK-5119 Update apache velocity by suubi-joshua · Pull Request #4310 · openmrs/openmrs-core · GitHub