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 size very small, ran build, saw logs, verified small logs in database.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10457

Differential Revision: https://secure.phabricator.com/D15375