Use thread-safe FastDateFormat in TsUtils.scala

[divyamanohar-stripe]

Summary

Use FastDateFormat instead of SimpleDateFormat in TsUtils.scala

Why / Goal

We recently introduced some parallelizations that included this logging in SawtoothOnlineAggregator.scala. Since SimpleDateFormat is not thread-safe, we ran into some race conditions with these logs. In order to resolve this, we can instead use the thread-safe FastDateFormat

Test Plan

• Covered by existing CI
• Tested in our QA environment

Reviewers

[nikhilsimha]

[piyushn-stripe]

Thanks for fixing!