Refactor Core Data application layer code

[lindseysimple]

🚀 Feature Request

Relevant Package [REQUIRED]

This feature request is for Core Data code refactoring.

Description [REQUIRED]

A clear and concise description of the problem or missing capability...

1. Synchronized the Core Data application layer code to refactor the functions to be the struct methods of CoreDataApp.

// CoreDataApp encapsulates the Core Data Application functionality
// TODO: Extend this App usage beyond Events.
type CoreDataApp struct {
	lc                       logger.LoggingClient
	eventsPersistedCounter   gometrics.Counter
	readingsPersistedCounter gometrics.Counter
}

2. Currently the application/reading.go file has grown too large and difficult to maintain, extract the event purging related code into a separate file.

Describe the solution you'd like

Same as described in the Description section.