connect: fix connection shutdown for event based processing

connect: fix connection shutdown for event based processing

connections being shutdown would register sockets for events, but then
never remove these sockets again. Nor would the shutdown effectively
been performed.

- If a socket event involves a transfer, check if that is the
  connection cache internal handle and run its multi_perform()
  instead (the internal handle is used for all shutdowns).
- When a timer triggers for a transfer, check also if it is
  about the connection cache internal handle.
- During processing shutdowns in the connection cache, assess
  the shutdown timeouts. Register a Curl_expire() of the lowest
  value for the cache's internal handle.

Reported-by: Gordon Parke
Fixes #14280
Closes #14296