multi: make add_handle free any multi_easy

multi: make add_handle free any multi_easy

If the easy handle that is being added to a multi handle has previously
been used for curl_easy_perform(), there is a private multi handle here
that we can kill off. While it flushes some caches etc for the easy
handle would it be used for an easy interface transfer again after being
used in the multi stack, this cleanup simplifies behavior and uses less
memory.

Closes #12992