Rename the field session_name to service_instance_name

[vaibhav-kaushal]

The field session_name in the database (and the corresponding struct fields in the code) actually refer to the instance of the service that sent the log message to the server. The current name session_name is ambiguous, especially when we are talking about user login session and similar stuff. We should rename it to something which makes more sense, e.g. service_instance_name would be a good choice.