This commit creates a CustomFormatter once, outside the decoding loop,
and passes a reference to that formatter to PqDecoder::decode_message.

API change: the PqDecoder::decode_message method accepts a
CustomerFormatter as its third argument rather than a bool.

This is in preparation for making pq multi-threaded; the trait `Read`
is not `Send` and so with the previous declaration (read: &'a mut
Read) ByteConsumer was _not_ Send and could not be moved into a
closure, i.e., with thread::spawn.

This commit modifies the API of ByteConsumer by making it take
ownership of a value of type T where T implements the Read trait.

This commit adds concurrency to the decoding part of
CommandRunner::decode_or_convert.  A pair of threads -- a producer and
a consumer -- are spawned; the producer decodes Protobuf values and
sends the decoded Value over a channel to the consumer who encodes the
values to JSON and outputs them to stdout.