(TK-37) Increase default request buffer size and make request header size configurable

This PR does two things. First off, it bumps the default request buffer size as per ticket TK-37. In writing tests for this, however, I discovered that Jetty puts a limit on the size of the request header less than the new default value of the request buffer size, and if a header exceeds this limit, Jetty will return an HTTP 413 error regardless of whether or not the request buffer overflows. As a result, this PR has changed the default request size to be equal to the default request buffer size, and has also made it possible to configure the request size in a trapperkeeper configuration file.
I'd phrase this as "maximum size for an HTTP request header".  I'd also take out the second line as this value might need to be bumped up for other reasons not related to a cookie, e.g., some long token in an "Authorization" header.