Improve Avro schema generation

In SQOOP-2252 we've added default value to all Avro schemas - NULL as by default we're creating all columns in Avro as nullable. I think that this can be further improved:

• Instead of failing back to allow all columns to contain NULL, we should propagate this information from table itself.
• We could also propagate the column's default value from database.
• We might even add parameter --map-column-avro-default that will enable to specify any other default value (similar to --map-column-java argument).