reduce and localize constant string use
Literal String#freeze avoids allocations since Ruby 2.1 via the
opt_str_freeze instruction, so we can start relying on it in
some places as Ruby 2.1 adoption increases. The 100-continue
handling is a good place to start since it is an uncommonly-used
code path which benefits from size reduction and the negative
performance impact is restricted to a handful of users.
HTTP_RESPONSE_START can safely live in http_request.rb as its
usage does not cross namespace boundaries
The goal is to eventually eliminate Unicorn::Const entirely.