Add configuration for running health checks without threads

[mnitchie]

Addresses #360

A configuration option to allow for running health checks without spinning up threads would be useful in a few different scenarios.

This adds a HEALTH_CHECK["DISABLE_THREADING"] config to support this. The config defaults to False so existing consumers of the lib will still work the same way.

h/t @ActionScripted

[dimaqq]

@ilyamonakov review pls

[rapkyt]

Can this be merged? I have randoms time outs only for the health check endpoint that I think it will be solved by not using threading, thanks!

[mnitchie]

@frankwiles and/or @jefftriplett - thanks for your work on this repo! Curious what you think of this and if it could be merged.

[frankwiles]

Yeah I think we can merge this. I didn't even remember it was using threads, but can definitely see that getting in the way.

My plan/hope for v2 (when I can find the time for it) is for there to be a separate view that will run the checks async for a nice UI/UX display, but keep them sync obviously for automated use cases (i.e. load balancers knowing which are healthy, etc)

[frankwiles]

I'll get a release put together in a couple of days.

[ron8mcr]

I'll get a release put together in a couple of days.

@frankwiles Hope that #397 will be included to this release too