Refactor ACS heartbeat message handling

[danehlim]

Summary

Refactor ACS heartbeat message handling and update other associated code.

Implementation details

0. Remove RegisterResponder method from wsclient.RequestResponder interface as it is unnecessary and will not be required for any future responders (see this comment thread for more details)
   • ecs-agent/wsclient/client.go
     • Remove RegisterResponder method and associated comments
   • ecs-agent/wsclient/mock/client.go
     • Regenerate mocks after removal performed above
1. Refactor ACS heartbeat message handling and move to ecs-agent/
   • agent/acs/handler/heartbeat_handler.go -> ecs-agent/acs/session/heartbeat_responder.go
     • Define heartbeatResponder struct and refactor existing code, such that heartbeatResponder struct implements the wsclient.RequestResponder interface
     • Use github.com/aws/amazon-ecs-agent/ecs-agent/logger instead of github.com/cihub/seelog for logging
   • agent/acs/handler/heartbeat_handler_test.go -> ecs-agent/acs/session/heartbeat_responder_test.go
     • Update tests accordingly based off of above changes
2. Update other associated files
   • agent/acs/handler/acs_handler.go
     • Initialize heartbeat responder and add its HandlerFunc function as a request handler to the ACS client, such that heartbeat responder dictates what to do in response to receiving heartbeat messages from ACS
       • Eventually, each ACS message type will have its own responder (which implements the wsclient.RequestResponder interface) that dictates what to do in response to ACS messages of that specific type
   • ecs-agent/logger/field/constants.go
     • Add new MessageID constant to logger constants, since we expect this to be commonly used in logging as we continue to refactor handling of other ACS messages in future pull requests

Testing

Unit, integration, and functional tests.

Description for the changelog

Refactor ACS heartbeat message handling

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.