[BUG] session: release slot before processing pending connections

[BUG] session: release slot before processing pending connections

When a connection error is encountered on a server and the server's
connection pool is full, pending connections are not woken up because
the current connection is still accounted for on the server, so it
still appears full. This becomes visible on a server which has
"maxconn 1" because the pending connections will only be able to
expire in the queue.

Now we take care of releasing our current connection before trying to
offer it to another pending request, so that the server can accept a
next connection.

This patch should be backported to 1.4.