It might not work if there were more than one "access_log" directives
pointed to the same file and duplicate buffer parameters.

The new directives {proxy,fastcgi,scgi,uwsgi,memcached}_next_upstream_tries
and {proxy,fastcgi,scgi,uwsgi,memcached}_next_upstream_timeout limit
the number of upstreams tried and the maximum time spent for these tries
when searching for a valid upstream.

Since peer.tries is never reset it can now be limited if required.

The "single" parameter is deprecated and ignored since 5b5c07dee156 (1.3.2).

In fastcgi, scgi and uwsgi modules there are no default cache keys, and
using a cache without a cache key set is likely meaningless.

When memory allocation failed in ngx_http_upstream_cache(), the connection
would be terminated directly in ngx_http_upstream_init_request().
Return a INTERNAL_SERVER_ERROR response instead.

The etag->hash must be set to 0 to avoid an empty ETag header being
returned with the 500 Internal Server Error page after the memory
allocation failure.

Reported by Markus Linnala.

The messages "ngx_slab_alloc() failed: no memory in cache keys zone"
from the file cache slab allocator are suppressed since the allocation
is likely to succeed after the forced expiration of cache nodes.
The second allocation failure is reported.

In theory, this can provide a bit better distribution of latencies.

Also it simplifies the code, since ngx_queue_t is now used instead
of custom implementation.