Historically, it was possible to match either a file or directory
in the following configuration:

    location / {
        try_files $uri/ =404;
    }

If use_temp_path is set to off, a subdirectory "temp" is created in the cache
directory.  It's used instead of proxy_temp_path and friends for caching
upstream response.

Keeps time spent on obtaining the header from an upstream server.  The value is
formatted similar to the $upstream_response_time variable.

When set to "off", temporary files for cacheable responses will be stored
inside cache directory.

Some parts of code related to handling variants of a resource moved into
a separate function that is called earlier.  This allows to use cache file
name as a prefix for temporary file in the following patch.

No functional changes.

The configuration handling code has changed to look similar to the proxy_store
directive and friends.  This simplifies adding variable support in the following
patch.

No functional changes.

Currently, storing and caching mechanisms cannot work together, and a
configuration error is thrown when the proxy_store and proxy_cache
directives (as well as their friends) are configured on the same level.

But configurations like in the example below were allowed and could result
in critical errors in the error log:

    proxy_store on;

    location / {
        proxy_cache one;
    }

Only proxy_store worked in this case.

For more predictable and errorless behavior these directives now prevent
each other from being inherited from the previous level.

This changes internal API related to handling of the "store"
flag in ngx_http_upstream_conf_t.  Previously, a non-null value
of "store_lengths" was enough to enable store functionality with
custom path.  Now, the "store" flag is also required to be set.

No functional changes.