ECS agent to acknowledge server heartbeat messages

[LiangChiAmzn]

Summary

This PR adds the support for ECS agent to acknowledge server heartbeat messages. This would help both ECS agent and ECS backend server to detect stale socket connections, then take proactive actions to handle it.

Implementation details

ECS server will soon start to actively clean up stale connections if ECS agent is not acknowledging heartbeat messages. Since such change is backward-incompatible, a new protocolVersion is introduced for ECS agent to control the behavior from client side when establishing the WebSocket connection.

If the version is set as 2, then the ECS agent must acknowledge all server heartbeats. Null protocol version is treated as 1.

Testing

make test
make release

Manually tested the change by building the agent, connect to ECS, then inspect ECS backend logs:

• Positive case: connection is healthy when protocol version = 2 and heartbeats are being acked
• Negative case: ECS will cut the connection if protocol version = 2 but heartbeat acks are missing

New tests cover the changes: yes

Description for the changelog

Enhancement - Add support for ECS agent to acknowledge server heartbeat messages

Licensing

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

[fenxiong]

lgtm other than cam's comment

[sparrc]

lgtm assuming tests pass