NerscMover sync hsi command blocks heartbeats

[blinkdog]

Like it says on the tin, when running the hsi command to write a file to HPSS, the command is blocking and prevents heartbeats from being sent to the LTA DB back home.

Perhaps we need to un-block the hsi command, along these lines?
https://fredrikaverpil.github.io/2017/06/20/async-and-await-with-subprocesses/