Cleanup Cache::generate_key() function

Remove BROKEN_HACK_FOR_VARY_ON_UA macro and deprecated code
in Cache::generate_key().

Also remove the unused *request* parameter.
Fix bad url_md5 before marking Owner-Left-Time

Getting the url_md5 from ->first_key is wrong when cache belongs to
another node.

Just using cache_sm->lookup_url to generate key can work well.

This patch also refines some logs output and do a little optimization.

Signed-off-by: Yunkai Zhang <qiushu.zyk@taobao.com>