Imp: service provider improvement todo list

[u0x01]

service provider improvement todo list:

• allow service provider return nil response

func (p *Provider) GetUser(userID int64) (*User, error) {
	...
	return nil, nil
}

• allow service provider none response

func (p *Provider) SetUserInfo(userInfo *User) error {
	...
	return nil
}

• allow service provider none request

func (p *Provider) TakeUser() (*User, error) {
	...
	return &User{}
}

• host bind ip auto detect when not being configured
• provider config map abstractions

conMap, _ := config.Load()
res, err := conMap["com.ikurento.user.UserProvider"].GetRPCService("com.ikurento.user.UserProvider").(*UserProvider).Echo(context.TODO(), "OK")
if err != nil {
	panic(err)
}

to

providerInstance, _ := config.Build()
res, err := providerInstance.GetRPCService("com.ikurento.user.UserProvider").(*UserProvider).Echo(context.TODO(), "OK")
if err != nil {
	panic(err)
}