Actuator Endpoint to Delete Sessions

[rwinch]

It would be nice to add an actuator Endpoint that:

• Allows finding sessions by username
• Displays the resulting sessions
• Then allows deleting the session

I think an HTML version of this would be awesome for administrators, but we would likely want to support all the various Actuator endpoint styles (i.e. JMX, JSON, etc)

Related #408

[vpavic]

This is scheduled to be implemented in Boot 2.0 via spring-projects/spring-boot#8342.