Zabbix

.......... [DEV-2263] moved agent2 postgresql logic into plugin

* commit '0757eeaa360b8461c00cf4ae771bea0fd77bd357':
  .......... [DEV-2263] updated plugin-support dependency
  .......... [DEV-2263] removed unused dependencies gopkg.in/yaml.v2, github.com/stretchr/testify
  .......... [DEV-2263] removed unused file left by mistake
  .......... [DEV-2263] removed tlsconfig from agent2; updated go.mod for agent2
  .......... [DEV-2263] converted built-in postgresql plugin to standalone plugin