BUG/MINOR: dns: prevent ds accumulation within dss

BUG/MINOR: dns: prevent ds accumulation within dss

when dns session callback (dns_session_release()) is called upon error
(ie: when some pending queries were not sent), we try our best to
re-create the applet in order to preserve the pending queries and give
them a chance to be retried. This is done at the end of
dns_session_release().

However, doing so exposes to an issue: if the error preventing queries
from being sent is still encountered over and over the dns session could
stay there indefinitely. Meanwhile, other dns sessions may be created on
the same dns_stream_server periodically. If previous failing dns sessions
don't terminate but we also keep creating new ones, we end up accumulating
failing sessions on a given dns_stream_server, which can eventually cause
ressource shortage.

This issue was found when trying to address ("BUG/MINOR: dns: add tempo
between 2 connection attempts for dns servers")

To fix it, we track the number of failed consecutive sessions for a given
dns server. When we reach the threshold (set to 100), we consider that the
link to the dns server is broken (at least temporarily) and we force
dns_session_new() to fail, so that we stop creating new sessions until one
of the existing one eventually succeeds.

A workaround for this fix consists in setting the "maxconn" parameter on
nameserver directive (under resolvers section) to a reasonnable value so
that no more than "maxconn" sessions may co-exist on the same server at
a given time.

This may be backported to all stable versions.
("CLEANUP: dns: remove unused dns_stream_server struct member") may be
backported to ease the backport.

(cherry picked from commit 5288b39011b2449bfa896f7932c7702b5a85ee77)
Signed-off-by: Willy Tarreau <w@1wt.eu>