CRUNCH-297: Add parallelism options for joins/cogroups to the Scrunch API
CRUNCH-298: Slim down FormatBundle serialization
CRUNCH-293: Add AvroMode to inject avro readers.

AvroMode is an enum that is used to construct DatumReaders and
DatumWriters. The default generic, specific, and reflect implementations
returned can be overridden cleanly.