BUG/MEDIUM: mux-spop: Wait end of handshake to declare a spop connection ready

BUG/MEDIUM: mux-spop: Wait end of handshake to declare a spop connection ready

A SPOP connection must not be considered as ready while the hello handshake
is not finished with success. In addition, no error or shutdown must have
been reported for the underlying connection. Otherwise a freshly openned
spop connexion may be reused while it is in fact dead, leading to a
connection retry.

This patch must be backported to 3.1.

(cherry picked from commit fd7ebf117bdefdcfbd1c30a079585867eb22a57d)
Signed-off-by: Willy Tarreau <w@1wt.eu>