Add a "--rate" flag to bin/harbormaster write-log to support testing live log streaming

Summary: Depends on D19151. Ref T13088. While dramatically less exciting than using lolcat and less general than pv, this should do the job adequately.

Test Plan: Piped a sizable log into bin/harbormaster write-log with --rate 2048, saw a progress bar. Loaded the log in the web UI and saw it grow as the page reloaded.

Reviewers: yelirekim

Reviewed By: yelirekim

Subscribers: yelirekim, PHID-OPKG-gm6ozazyms6q6i22gyam

Maniphest Tasks: T13088

Differential Revision: https://secure.phabricator.com/D19152