BUG/MAJOR: http: don't requeue an idle connection that is already queued

BUG/MAJOR: http: don't requeue an idle connection that is already queued

Cyril Bonté reported a reproduceable sequence which can lead to a crash
when using backend connection reuse. The problem comes from the fact that
we systematically add the server connection to an idle pool at the end of
the HTTP transaction regardless of the fact that it might already be there.

This is possible for example when processing a request which doesn't use
a server connection (typically a redirect) after a request which used a
connection. Then after the first request, the connection was already in
the idle queue and we're putting it a second time at the end of the second
request, causing a corruption of the idle pool.

Interestingly, the memory debugger in 1.7 immediately detected a suspicious
double free on the connection, leading to a very early detection of the
cause instead of its consequences.

Thanks to Cyril for quickly providing a working reproducer.

This fix must be backported to 1.6 since connection reuse was introduced
there.