BUG: http: do not abort keep-alive connections on server timeout

BUG: http: do not abort keep-alive connections on server timeout

When a server timeout is detected on the second or nth request of a keep-alive
connection, HAProxy closes the connection without writing a response.
Some clients would fail with a remote disconnected exception and some
others would retry potentially unsafe requests.

This patch removes the special case and makes sure a 504 timeout is
written back whenever a server timeout is handled.

Signed-off-by: lsenta <laurent.senta@gmail.com>