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When the async progress thread blocked the progress engine and yielded
control, if a thread started waiting inside a wait routine, e.g.,
ADIOI_GEN_iwc_wait_fn, of NBC I/O implementation, a deadlock happened.
The thread waiting continuously called MPI_Test to make progress, but
the progress engine did not make progress because it was blocked due to
the async progress thread.  The async progress thread tried to acquire
the lock, but the waiting thread did not release the lock because it
did not finish the wait routine.  Thus, it was a deadlock. This patch
fixes this deadlock problem by forcing the waiting thread to yield if
the progress engine has been blocked by another thread.

Fixes #2202
Signed-off-by: Rob Latham <robl@mcs.anl.gov>