Problem

Finagle's HTTP Message class has getters/setters for several frequently used header fields. However, some of those fields are only valid in requests, and others are only valid in responses. There are examples of mistaken use of these methods to retrieve request-only headers from a response and vice versa. It increases chances for programmer confusion.

Solution

Move getters/setters for request-only headers into the Request subclass and those for response-only headers in the the Response subclass.

JIRA Issues: CSL-11981

Differential Revision: https://phabricator.twitter.biz/D890075