The number of I/O threads.
ZmqException if $(ZMQ) reports an error.
$(ZMQREF zmq_ctx_get()) and $(ZMQREF zmq_ctx_set()) with ZMQ_IO_THREADS.
auto ctx = Context(); ctx.ioThreads = 3; assert (ctx.ioThreads == 3);
See Implementation
The number of I/O threads.