dist-array/DA-threads example has a thread-safety bug

[Dan Bonachea]

dist-array/DA-threads is currently using a global promise<> p; to signal between worker threads and master thread.

upcxx::promise is not thread-safe, so this is not guaranteed to work. The likely failure mode is a hang at runtime.

[Daniel Waters]

Pull request #22 from cache-opt branch allows DA-threads test to pass