Streaming logs

[ejholmes]

https://app.asana.com/0/58153350582042/58211982755442/f

Closes https://github.com/remind101/conveyor/issues/15

This adds streaming logs backed by CloudWatch Logs.

TODO

• [x] Use some binary sequence when closing the log stream, so that readers can close.
• [x] Set log retention to 7 days.
• [x] Lock down ACL to only allow access to the provisioned log group.

[phobologic]

:+1: