add english document of consistent_hash module

修正一致性哈希中文文档，使其语句更加通顺(方便翻译)
同时增加了一致性哈希的英文文档

update chinese document for consistent_hash module
add english document for consisitent_hash module

For human-readable markdown, see https://github.com/chobits/tengine/blob/consistent_hash_doc/docs/modules/ngx_http_upstream_consistent_hash_module.md