Introduce a built-in variable: $normalized_request

When working with key-value store, such as memcached, we usually use URL
to generate unique key for each object.

But unfortunately, each URL may have several forms, for example:

1) http://abc.com/search?text=Hello+G%C3%BCnter#h1
2) http://ABC.COM/search?text=Hello+G%c3%bcnter#h1
3) HTTP://abc.com/search?text=Hello+G%C3%BCnter#h3

All above froms are refer to the same object. So we have to normalize URL
before generating object key.

As we known, URL syntax looks like:
```
<scheme>://<user>:<password>@<host>:<port>/<path>;<params>?<query>#<frag>
```

The $normalized_request variable is the normalized form of URL, which will:
 - exclude ```#<frag>``` part from URL.
 - convert ```<scheme>``` and ```<host>``` to lower-case.
 - convert all escape characters(%nn) in ```<path>``` to upper-case.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>
Could we use lower-case instead? it's more popular in the nginx code.