Adding support for JSON serialization of SenseiRequest and SenseiResult

This should be all that's necessary to add support for SenseiRequest. Lei - I couldn't get it to build because I needed a new bobo, but the new bobo wasn't passing tests for me. Could you test this?

We should test performance of this. Also, we may want to LZO compress the SenseiResult JSON for space savings.To setup a lab, just change the sensei broker to use CoreSenseiServiceImpl.JSON_SERIALIZER in it's constructor. Be warned that doing it in one shot will make the broker incompatible with the searchers, so we should do a release to add the new serialization and then another release to turn it on.

With a little work, we could make the serialization class completely configurable from the broker side, which might make it easier to deploy.