(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.
Would be nice to reference the `default-request-size` def that you added in `jetty9-config.clj` here so we aren't listing the same value in two places.