terminology refactor: header

[vinniefalco]

We use the term "message" to describe the start-line plus fields, and this is confusing because we also use "message" to describe the full request or response including the body. I propose the following, which we should discuss first:

1. Repurpose the term "header" to mean start-line plus fields
2. Always say "fields" instead of "headers" (never say "headers")
3. Use "request header" to mean a request-line plus fields
4. Use "response header" to mean a status-line plus fields
5. Rename message_base to header