Trn 368

Original ticket reads:

Basically, TRN-126, but with headers and not just the HTTP body. Ideally these would be stored in a way that makes them easy to work with in an automated way (i.e. not as a big string or something). Cassandra's map type may be best for this.
This should also be orthogonal to storing the request body. We may want to store headers but not store the request body.