memcached_next_upstream and memcached_next_upstream_tries support

[helmus]

When using multiple servers it would be useful if memc_next_upstream could be supported to result in less cache misses. The native memcached module in openresty supports these but not memc which got me confused for a little while:

for reference: http://nginx.org/en/docs/http/ngx_http_memcached_module.html#memcached_next_upstream http://nginx.org/en/docs/http/ngx_http_memcached_module.html#memcached_next_upstream_tries

Thanks for maintaining such an awesome library !