AVRO-3504: New method "skip_data" for DataFileReader

[izveigor]

DatumReader has method "skip_data" which skips data with the help of a decoder. I suggest applying this method for DataFileReader to skip a lot of useless data.

New method "skip_data" for DataFileReader would accept the argument "number" which means number of datums that need skip.

Test for new fixture was written in "test_datafile.py". The test removes 50 misses from coverage.

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