Move reactor master-clients to threads

Re-do of #18254 (which was reverted in #18741)

Looks like the errors seen where unrelated. I checked with @whiteinge and neither of us were able to reproduce the error (details in #18254)

In addition to that I found a bug with malformed SLS files causing the reactor process to die.
Would love to see a debug log with `exc_info=True` here.