Patrick Griffis authored Jan 23, 2020 and Philip Withnall committed Feb 14, 2020

This is a fairly large refactoring. The highlights are:

- Removing in-progress connections/addresses from GSocketClientAsyncConnectData:

  This caused issues where multiple ConnectionAttempt's would step over eachother
  and modify shared state causing bugs like accidentally bypassing a set proxy.

  Fixes #1871
  Fixes #1989
  Fixes #1902

- Cancelling address enumeration on error/completion

- Queuing successful TCP connections and doing application layer work serially:

  This is more in the spirit of Happy Eyeballs but it also greatly simplifies
  the flow of connection handling so fewer tasks are happening in parallel
  when they don't need to be.

  The behavior also should more closely match that of g_socket_client_connect().

- Better track the state of address enumeration:

  Previously we were over eager to treat enumeration finishing as an error.

  Fixes #1872
  See also #1982

- Add more detailed documentation and logging.

Closes #1995