BUG/MEDIUM: backend: always release the previous connection into its own target srv_list

BUG/MEDIUM: backend: always release the previous connection into its own target srv_list

There was a bug reported in issue #19 regarding the fact that haproxy
could mis-route requests to the wrong server. It turns out that when
switching to another server, the old connection was put back into the
srv_list corresponding to the stream's target instead of this connection's
target. Thus if this connection was later picked, it was pointing to the
wrong server.

The patch fixes this and also clarifies the assignment to srv_conn->target
so that it's clear we don't change it when picking it from the srv_list.

This must be backported to 1.9 only.