support set default value when scrape failed

[suninuni]

• 根据 tomasen/fcgi_client 未 merge 的 PR，将 fcgi_client 改成了 github.com/kanocz/fcgi_client 的版本以支持设置超时，目前设置的是 2s。
• scrape failed 时默认会返回 phpfpm_scrape_failures 和 phpfpm_up 来标识 scrape 失败，因此在此基础上加上了 --phpfpm.default-active-processes 用于设置失败时的 phpfpm_active_processes。

# HELP phpfpm_active_processes The number of active processes.
# TYPE phpfpm_active_processes gauge
phpfpm_active_processes{pool="www",scrape_uri="tcp://127.0.0.1:9000/status"} 100
# HELP phpfpm_scrape_failures The number of failures scraping from PHP-FPM.
# TYPE phpfpm_scrape_failures counter
phpfpm_scrape_failures{pool="www",scrape_uri="tcp://127.0.0.1:9000/status"} 15
# HELP phpfpm_up Could PHP-FPM be reached?
# TYPE phpfpm_up gauge
phpfpm_up{pool="www",scrape_uri="tcp://127.0.0.1:9000/status"} 0

[suninuni]

@inetfuture please review and merge