R1:21ddfe442e67

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…