v3.0.0 - Node v12 to v16, fix `docker exec` communication

• Drop Node v8. Node v10 is unsupported, but tests still pass.
• Support Node v12, v14, and v16. The current release is also tested.
• Fix two-way communication with Docker exec process (issue #34)
• Fix closing of the socket to the Docker exec process
• Change debug log prefix from docker-exec-server to docker-exec-websocket-server
• Update runtime dependencies, including:
  • lodash 4.17.19 to 4.17.21
  • through2 3.0.1 to 4.0.2
  • ws 7.1.2 to 8.4.2
• Update development dependencies, including:
  • eslint 6.4.0 to 8.8.0, includes v12 syntax
  • mocha 6.2.0 to 9.2.0, includes moving settings to package.json
• Use Taskcluster community-tc and docker-compose for PR testing
• In tests, fully close opened resources
• Remove Vagrant, TravisCI, and container creation duplicated by mocha.