add lock to spirack driver

[peendebak]

@mtiggelman @Rubenknex

[mtiggelman]

Shouldn't the lock also be added to the 'write_data' function?

[peendebak]

@mtiggelman Maybe. That depends on whether the write function in the serial.Serial is thread safe (e.g. an atomic write). The write function uses win32.WriteFile, which seems to be thread safe (https://msdn.microsoft.com/en-us/library/windows/desktop/aa365747(v=vs.85).aspx)

We could add locks just to be safe.

[mtiggelman]

@peendebak I think you're right, then it's not necessary. Just the lock on the read_data should be fine then.