AvroIO does not handle partial reads

Reported by a user using HDFS with AvroIO, they are hitting an exception here: https://github.com/apache/beam/blob/4abb9cbc1db220af3663ddbacbbe26d7d443e621/sdks/java/core/src/main/java/org/apache/beam/sdk/io/AvroSource.java#L669

Looks like this is because we are assuming the read will always return the right number of bytes. We should be using IOUtils.readFully instead.