BUG/MAJOR: connection: prevent double free if conn selected for removal

BUG/MAJOR: connection: prevent double free if conn selected for removal

Always try to remove a connexion from its toremove_list in conn_free.
This prevents a double-free in case the connection is freed but was
already added in toremove_list.

This bug was easily reproduced by running 4-5 runs of inject on a
single-thread instance of haproxy :

$ inject -u 10000 -d 10 -G 127.0.0.1:20080

A crash would soon be triggered in srv_cleanup_toremove_connections.

This does not need to be backported.