MAJOR: backend: initial work towards connection reuse

MAJOR: backend: initial work towards connection reuse

In connect_server(), if we don't have a connection attached to the
stream-int, we first look into the server's idle_conns list and we
pick the first one there, we detach it from its owner if it had one.
If we used to have a connection, we close it.

This mechanism works well but doesn't scale : as servers increase,
the likeliness that the connection attached to the stream interface
doesn't match the server and gets closed increases.