Upstream: simplified proxy_store and friends configuration code.

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.
Upstream: mutually exclusive inheritance of "cache" and "store".

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.
Upstream: refactored proxy_cache and friends.

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.
Upstream: preset some cache configuration when bypassing.

No functional changes.
Upstream: added variables support to proxy_cache and friends.