2.11.0: Early hints, BC fix

• Merged PR #80: Fixed usage of "&" chars in multipart parameters
  (@johannes85)
• Fixed compatibility with older versions of xp-framework/networking
  without asynchronous server support. This lead to the server being
  started but not answering any request, see issue #79.
  (@thekid)
• Added support for HTTP status code 103 "Early Hints", see RFC 8297 and
  https://evertpot.com/http/103-early-hints. Note that this does not work
  in the development webserver, see php/php-src#7025.
  (@thekid)