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.
@jacksontj how would you feel about a debug log entry here of the 'Executing job <blah>' variety? For executions that go out to the minions it's easy to see the publish in the debug output, but for runner/wheel modules there is nothing in the logs to indicate that anything happened.