wireproto: add config knob for http header length limit Well-behaved Mercurial clients will respect the httpheader capability by not sending http headers longer than the given limit in bytes. The limit is currently hard-coded at 1024 bytes, a safe value for any web server. Since parsing headers is a notable factor in web server performance, tuning header size can nontrivially improve performance for request-heavy operations (eg. obsolete marker negotiation). Exposing the maximum header length limit as a configuration setting is a simple way to enable such tuning.