Use std::string_view in CookieParser::before_handle

[Sightem]

The current CookieParser::before_handle implementation creates several std::string objects during the parsing of the Cookie header value. This can be optimized by using std::string_view to slice the header string directly.

Addresses the todo at:

Crow/include/crow/middlewares/cookie_parser.h

Line 234 in e8b1251

// TODO(dranikpg): remove copies, use string_view with c++17