R1:0daa9ad98758

Use PhutilRope as a buffer in Harbormaster BuildLogs

Summary:
Ref T10457. Currently, every `append()` call necessarily generates queries, and these queries are slightly inefficient if a large block of data is appended to a partial log (they do about twice as much work as they technically need to).

Use `PhutilRope` to buffer `append()` input so the logic is a little cleaner and we could add a rule like "flush logs no more than once every 500ms" later.

Test Plan:
- Ran a build, saw logs.
- Set chunk…