BUG/MEDIUM: backend: prefer-last-server breaks redispatch

BUG/MEDIUM: backend: prefer-last-server breaks redispatch

Since 1.5-dev20, we have a working server-side keep-alive and an option
"prefer-last-server" to indicate that we explicitly want to reuse the
same server as the last one. Unfortunately this breaks the redispatch
feature because assign_server() insists on reusing the same server as
the first one attempted even if the connection failed to establish.

A simple solution consists in only considering the last connection if
it was connected. Otherwise there is no reason for being interested in
reusing the same server.