BUG/MINOR: stream: don't force retries if the server is DOWN

BUG/MINOR: stream: don't force retries if the server is DOWN

Arkadiy Kulev noticed that if a server is marked down while a connection
is being trying to establish, we still insist on performing retries on
the same server, which is absurd. Better perform the redispatch if we
already know the server is down. Because of this, it's likely that the
observe-l4 and sudden-death mechanisms are not optimal an cannot help
much the connection which was used to detect the problem.

The fix should be backported to 1.6 and 1.5 at least.