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'
all of the tests in this file might benefit from some longer and more varied test strings and messages. right now i'm only convinced that this works for keys and vals of length 1 and the message MSG.

might be a good place to apply generative testing - let me know if you want to talk through how to do that