MEDIUM: backend: don't call si_alloc_conn() when we reuse a valid connection

MEDIUM: backend: don't call si_alloc_conn() when we reuse a valid connection

connect_server() already does most of the check that is done again in
si_alloc_conn(), so let's simply reuse the existing connection instead
of calling the function again. It will also simplify the connection
reuse.

Indeed, for reuse to be set, it also requires srv_conn to be valid. In the
end, the only situation where we have to release the existing connection
and allocate a new one is when reuse == 0.