Support datetime values

[mariusvniekerk]

Since avro 1.8.0 support for datetime has been added to the format. We should support reading that.

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[mariusvniekerk]

These may be harder to do since the C avro api does not support logical types.

[martindurant]

This could easily be done in dask_reader.read_avro_bytes, where there is a step to assign values from those loaded into the pre-allocated dataframe. (That doesn't solve the general problem for those not using the interface)
You can always assign int values directly into a numpy array of time type, but it would require multiplication by some factor to become the correct ns resolution.

[martindurant]

https://github.com/maxpoint/cyavro/pull/24/files#diff-7a8f47ca203140559f8ca8b671f51af1R278