Move ACS session to ecs-agent module and refactor

[danehlim]

Summary

Move ACS session to ecs-agent module and refactor associated code.

Implementation details

• agent/acs/handler/acs_handler.go -> ecs-agent/acs/session/session.go
  • Various code enhancement changes and making the ACS session more general such that it makes less assumptions about the underlying Agent making use of the session. Among the most notable are:
    • Session's Start method and other related methods it calls now take in a context.Context in favor of storing a context.Context as a struct field in the whole session itself
    • Session's Start method also now returns the error code returned by the context, which provides meaningful information about why the Session ended (as opposed to current behavior, where the method always returns nil)
    • Pass in the following pre-initialized as session struct fields instead of determining what they are from within the session itself: agentVersion, agentHash, dockerVersion, inactiveInstanceCB() function, PayloadMessageHandler, CredentialsMetadataSetter, ENIHandler, ResourceHandler, ManifestMessageIDAccessor, SequenceNumberAccessor, TaskComparer, TaskStopper, metrics.EntryFactory, and *wsclient.WSClientMinAgentConfig
• agent/acs/handler/acs_handler_test.go -> ecs-agent/acs/session/session_test.go
  • Update ACS session tests accordingly based off of above changes to ACS session
  • General cleanup/quality of life updates to tests (e.g., add additional tests, use assert package for test assertions instead of if statement blocks, use defined test constants instead of hard coding test values at the test level, remove initialization and usage of some unnecessary variables, update some code comments to be more clear, etc.)
• agent/app/agent.go
  • Make corresponding changes based off of above changes to ACS session (e.g., update what is passed into the session, how the return value of its Start method is handled)

Testing

Unit, integration, and functional tests.

New tests cover the changes: yes

Description for the changelog

Move ACS session to ecs-agent module and refactor

Licensing

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