Refactor Blocking I/O in MtlsUtils

[blakeli0]

There is a the Boq violation for MtlsUtils regarding blocking I/O when we try to migrate this repo to google3.
Even though it is not required to not having blocking I/O on Github and the code looks reasonable, we also try to avoid process spawning and blocking I/O.
Per @TimurSadykov, this code path also has a narrow use case so it may not be worth it to refactor it. Assigning to @TimurSadykov for further triaging and prioritizing.