CRUNCH-381 Use descriptions in parallelDo calls

Add simple descriptions to PCollection#parallelDo calls in library
methods so that it's easier to trace through job plans.
CRUNCH-382 Add DeepCopier for Avro ByteBuffers

Replace the NoOpDeepCopier for Avro ByteBuffers. ByteBuffers are
not immutable, and are reused within Avro, so places where
PType#getDetachedValue on Avro byte[] values will now work as
expected.