When got multiple upstream IP addresses using DNS resolving, the number of
upstreams tries and the maxinum time spent for these tries were not affected.
This patch fixed it.

The code is now similar to ngx_event_pipe_read_upstream().

The directives limit the upstream read rate.  For example,
"proxy_limit_rate 42" limits proxy upstream read rate to
42 bytes per second.

Spaces in Accept-Charset, Accept-Encoding, and Accept-Language headers
are now ignored.  As per syntax of these headers spaces can only appear
in places where they are optional.

If a variant stored can't be used to respond to a request, the variant
hash is used as a secondary key.

Additionally, if we previously switched to a secondary key, while storing
a response to cache we check if the variant hash still apply.  If not, we
switch back to the original key, to handle cases when Vary changes.

It replaces c->buf in checks in ngx_http_file_cache_open(), making it possible
to reopen the file without clearing c->buf.  No functional changes.

To cache responses with Vary, we now calculate hash of headers listed
in Vary, and return the response from cache only if new request headers
match.

As of now, only one variant of the same resource can be stored in cache.

The "proxy_ignore_header" directive now undersands the "Vary" parameter
to ignore the header as needed.