Refactor database client in core-metadata

[jduranf]

Should be fine to refactor database client in core-metadata using an interface like it's done in core-data and in export-client.

If it's fine, I can do it.

[feclare]

I added that interface in
https://github.com/feclare/edgex-go/tree/coremetadatadbinterface

I'm also refactoring the code to have all the database functions in its own package.

[jduranf]

@feclare Great!
Please, let me know if I can help in anything.

[tsconn23]

Closing this issue as per the email threads this morning, @feclare is well underway here.

[tsconn23]

Actually, will assign to @feclare so that he can associate with the relevant PR when ready.