* refactoring config

* Update by comments

* show what config frps uses (config file / command line)
* optimize the "start successfully" log message

* refactoring monitor code, support prometheus
* remove vendor

token is the current token comparison, and oidc generates oidc token using client-credentials flow. in addition - add ping verification using the same method

The ClientCommonConf, configuration file path, and server UDP port are
now passed around as arguments instead of being shared between
components as global variables. This allows for multiple clients to
exist in the same process, and allows client.Session to be used as a
library more easily.

This variable didn't seem to be used anyway, so no further changes were
required.

The ServerCommonConf is now passed around as an argument instead of
being shared between components as a global variable. This allows for
more natural interaction with server.Session as a library and allows for
multiple servers to co-exist within the same process.

Related: #1387

whether it's success or fail, otherwise it will cause memory leak
ref: https://golang.org/pkg/net/http/