multi: track users of a socket better

They need to be removed from the socket hash linked list with more care.

When sh_delentry() is called to remove a sockethash entry, remove all
individual transfers from the list first. To enable this, each Curl_easy struct
now stores a pointer to the sockethash entry to know how to remove itself.

Reported-by: Tom van der Woerdt and Kunal Ekawde

Fixes #3952
Fixes #3904
Closes #3953