multi: fix the transfer hashes in the socket hash entries

multi: fix the transfer hashes in the socket hash entries

- The transfer hashes weren't using the correct keys so removing entries
  failed.

- Simplified the iteration logic over transfers sharing the same socket and
  they now simply are set to expire and thus get handled in the "regular"
  timer loop instead.

Reported-by: Tom van der Woerdt
Fixes #4012
Closes #4014