Switch to broker.serializer instead of JSON-only solution

[asvetlov]

The code works well but contains a dirty hack for parsing pipeline data from task labels.

Pipeline loads()/dumps() are loadb()/dumpb() now.

Step's loads()/dumps() are gone away, seems like they don't override anything actually but all have the same code.

I'm not sure if the PR should be merged as is.

@s3rius please consider it as the idea demonstration.
If you agree with the code in general, I'll make bytes support to message labels first, and remove the hack later.

P.S. Sorry for two-week delay. Tests were hanging without any exception or log message. It took a while to figure out how to debug the problem.

[s3rius]

I also had no time to review it, sorry. Was busy as a bee on work.

[asvetlov]

I also had no time to review it, sorry. Was busy as a bee on work.

Don't rush please, a few days don't change the university :)