Possible crash when multiple threads call into `nixnet`

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nixnet initializes the ctypes functions per-call, like nidaqmx does. nidaqmx recently fixed a crash related to this.

See ni/nidaqmx-python@82f7ee5

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They fixed the crash by putting locks around initializing argtypes.

nimi-python reduces their risk by initializing the entire function once but doesn't completely eliminate it.
https://github.com/ni/nimi-python/blob/master/generated/nidmm/ctypes_library.py#L101

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Oh, and we should make sure to add stress tests for this case like DAQmx did.

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See also ni/nimi-python#92