keyval_decoder: convert 'MSG a=b c=d' to title and json

- support for quote-escaped values
- set payload to stringified json
- pass message fields via heka TOML
- ParseTitleAndKeyvals handles '\n'
If you use generative testing, I'd recommend an approach that Google talks about in [Paxos Made Live, section 6.3](http://www.cs.utexas.edu/users/lorenzo/corsi/cs380d/papers/paper2-1.pdf): log the seed you use for your random number generator, and provide some way to customize the seed used by the tests. That way, if you see a test failure from your generative tests, you can reproduce it deterministically by using the same seed to run the tests again.