It appears to be a relic from prototype locking removed in b0b7b5a35.

This makes it easier to understand why sessions may not be saved
in shared memory due to size.

All such transient buffers are converted to the single storage in BSS.

In preparation to raise the limit.

The "resolver" and "resolver_timeout" directives can now be specified
directly in the "upstream" block.

Specifying the upstream server by a hostname together with the
"resolve" parameter will make the hostname to be periodically
resolved, and upstream servers added/removed as necessary.

This requires a "resolver" at the "http" configuration block.

The "resolver_timeout" parameter also affects when the failed
DNS requests will be attempted again.  Responses with NXDOMAIN
will be attempted again in 10 seconds.

Upstream has a configuration generation number that is incremented each
time servers are added/removed to the primary/backup list.  This number
is remembered by the peer.init method, and if peer.get detects a change
in configuration, it returns NGX_BUSY.

Each server has a reference counter.  It is incremented by peer.get and
decremented by peer.free.  When a server is removed, it is removed from
the list of servers and is marked as "zombie".  The memory allocated by
a zombie peer is freed only when its reference count becomes zero.

Co-authored-by: Roman Arutyunyan <arut@nginx.com>
Co-authored-by: Sergey Kandaurov <pluknet@nginx.com>
Co-authored-by: Vladimir Homutov <vl@nginx.com>

Previously, the number of next_upstream tries included servers marked
as "down", resulting in "no live upstreams" with the code 502 instead
of the code derived from an attempt to connect to the last tried "up"
server (ticket #2096).

In TLSv1.3, NewSessionTicket messages arrive after the handshake and
can come at any time.  Therefore we use a callback to save the session
when we know about it.  This approach works for < TLSv1.3 as well.
The callback function is set once per location on merge phase.

Since SSL_get_session() in BoringSSL returns an unresumable session for
TLSv1.3, peer save_session() methods have been updated as well to use a
session supplied within the callback.  To preserve API, the session is
cached in c->ssl->session.  It is preferably accessed in save_session()
methods by ngx_ssl_get_session() and ngx_ssl_get0_session() wrappers.

This fixes inconsistency in what is stored in the "host" field.
Normally it would contain the "host" part of the parsed URL
(e.g., proxy_pass with variables), but for the case of an
implicit upstream specified with literal address it contained
the text representation of the socket address (that is, host
including port for IP).

Now the "host" field always contains the "host" part of the URL,
while the text representation of the socket address is stored
in the newly added "name" field.

The ngx_http_upstream_create_round_robin_peer() function was
modified accordingly in a way to be compatible with the code
that does not know about the new "name" field.

The "stream" code was similarly modified except for not adding
compatibility in ngx_stream_upstream_create_round_robin_peer().

This change is also a prerequisite for the next change.