Rename Stop*Receiver to Shutdown*Receiver

[ccaraman]

Shutdown() is the naming convention as detailed in the exporter spec

To keep naming consistent, update the receiver interface to Shutdown from Stop
https://github.com/open-telemetry/opentelemetry-service/blob/master/receiver/receiver.go#L53
https://github.com/open-telemetry/opentelemetry-service/blob/master/receiver/receiver.go#L73

[tigrannajaryan]

See #477 for deeper refactoring proposal.