switch ssntp payload from yaml to json for marshall/unmarshall

[kaccardi]

The yaml parser is much slower than the json parser. Switching our ssntp payload to be json rather than yaml will cut the marshal/unmarshal time in half.

[tpepper]

I wonder if this should be tagged "enhancement" versus eg: "technical debt cleanup"

[tpepper]

I've added the "janitorial" label on this. It's a modestly complex effort from an integration / validation standpoint, but in terms of code complexity and requisite initial skill to accomplish it should be a good starting point for a new contributor.

[markdryan]

Have we definitely agreed that it's a good idea to do this? IIRC the yaml parsing was slow but it wasn't on the critical path. Moving to json would make things less readable. Perhaps we could try to speed up the yaml parser.