Now we actually return NGX_ERROR on errors, and provide an error
string for memory allocation errors.

Reported by Markus Linnala.

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.

It's mostly dead code.  And the idea of thread support for this task has
been deprecated.

If a "resolver_timeout" occurs, only the first waiting request
was notified.  Other requests may hang forever.

RFC3986 says that, for consistency, URI producers and normalizers
should use uppercase hexadecimal digits for all percent-encodings.

This is also what modern web browsers and other tools use.

Using lowercase hexadecimal digits makes it harder to interact with
those tools in case when use of the percent-encoded URI is required,
for example when $request_uri is part of the cache key.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

The ngx_log_insert() function may invalidate pointer passed to it,
so make sure to don't use it after the ngx_log_insert() call.

DNS request resend on malformed responses was broken in 98876ce2a7fd (1.5.8).

Reported by Pramod Korathota.