Use asyncio subprocess interface in webservice code

Description

Using the blocking subprocess module in async code means stopping everything else while a subprocess runs. The equivalent asyncio interfaces allow other tasks to continue. This is a minor issue, as we're not using long-running subprocesses, but it cleans up some FIXMEs.

How Has This Been Tested?

It hasn't. I don't actually know how to get it set up to test. :-( Followed instructions from @danilevc to run it as xcaltst on max-exfl017 and trigger recalibration of some CALLAB data.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the code style of this project.

Reviewers

@danilevc @hammerd @ahmedk