http: move response_start_sent into the C ext
Combined with the previous commit to eliminate the `@socket'
instance variable, this eliminates the last instance variable
in the Unicorn::HttpRequest class.
Eliminating the last instance variable avoids the creation of a
internal hash table used for implementing the "generic" instance
variables found in non-pure-Ruby classes. Method entry overhead
remains the same.
While this change doesn't do a whole lot for unicorn memory usage
where the HttpRequest is a singleton, it helps other HTTP servers
which rely on this code where thousands of clients may be connected.