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'
Would the seed be changed every test run? If not, it seems like you limit the cases you will test. I do like the deterministic approach though.

An alternative approach is to simply log the generated input that creates the test failure. But then it's still non-deterministic.