Use a java Thread directly instead of core.async thread for worker execution

[mikub]

If a worker gets stuck on I/O when executing a workflow job it should be possible to terminate it. Switching to java.lang.Thread will give us ability to simply to interrupt the Thread.
See also #15

Currently workers are using core async thread:
https://github.com/mikub/titanoboa/blob/master/src/clj/titanoboa/processor.clj#L631