Slightly simplify the Harbormaster build log API

Summary:
Ref T5822. This prepares for inline compression and garbage collection of build logs.

This reduces the API surface area and removes a log from the "wait" step that would just log a message every 15 seconds. (If this is actually useful, I think we find a better way to communicate it.)

Test Plan:
Ran a build, saw a log:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5822

Differential Revision: https://secure.phabricator.com/D15371