Added proxy_upstream_tries directive, which can limit
proxy_next_upstream retry times. This directive can work in
main/server/http/location config, and is disabled by
default(default behavior is to try ALL upstreams). Partial code is
forked from patch: https://gist.github.com/shai-d/5446961.
Limit proxy/memcached/fastcgi/scgi/uwsgi upstream tries times.
Merge branch 'master' into limit_upstream_tries
add test cases for limit_upstream_tries
disable limit_upstream_tries when xxx_upstream_tries is set to 0.
Merge pull request #322 from supertcy/limit_upstream_tries

Limit proxy/memcached/fastcgi/scgi/uwsgi upstream tries times