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