Add admin APIs for managing the sessions in MySQL

[marpaia]

Once sessions are backed in MySQL, we will need to expose HTTP APIs so that the application can have session management functionality.

For example:

• Delete all sessions for a user (DELETE /api/v1/kolide/user/session)
• Delete a single session for a user (DELETE /api/v1/kolide/user/session/{id})

[marpaia]

#37