Allow registering existing callbacks

[abumq]

All the callbacks need to be modified in order to use existing callback (and if not provided create new as we're doing now to prevent breaking of existing code)

Essentially

template<typename T>
static inline bool installLoggerRegistrationCallback(const std::string& id) {

to

template<typename T>
static inline bool installLoggerRegistrationCallback(const std::string& id, T* cb = nullptr) {

This will allow us to function better in callbacks and allow us to import more stuffs.

Wait until #445 is merged since we are holding off any merges and waiting for @aparajita

[aparajita]

See #453. Go ahead and make the changes to that branch.

[easylogging]

We don't need this, we can set data for the callback using el::Loggers::loggerRegistrationCallback<T>()

Sample code:

#include "easylogging++.h"

INITIALIZE_EASYLOGGINGPP

class Data {
public:
    explicit Data(const std::string& name) : m_name(name) {}
    std::string name() const { return m_name; }
private:
    std::string m_name;
};

class Handler : public el::LoggerRegistrationCallback {
public:
    Handler() : m_data(nullptr) {}
    void setData(const Data* d) { m_data = d; }
protected:
    virtual void handle(const el::Logger* logger) {
       ELPP_COUT << "(Handler) Registered new logger " << logger->id() << " m_data = " << m_data->name() << std::endl;
    }
private:
    const Data* m_data;
};

void setup(Data* d) {
    const std::string configuratorName = "handler";
    el::Loggers::installLoggerRegistrationCallback<Handler>(configuratorName);
    Handler* h = el::Loggers::loggerRegistrationCallback<Handler>(configuratorName); // Get registered / installed callback with specified name
    h->setData(d);
}
int main(void) {

    Data d("mydd");
    setup(&d);

    LOG(INFO) << "Now we will register three loggers";

    el::Loggers::getLogger("logger1");
    el::Loggers::getLogger("logger2");
    el::Loggers::getLogger("logger3");
    return 0;
}

If we compile and run this
g++ test.cpp easylogging++.cc -o bin/test.cpp.bin -std=c++11

We get

$ ./bin/test.cpp.bin
2017-01-11 14:56:02,474 INFO  [default] Now we will register three loggers
(Handler) Registered new logger logger1 m_data = mydd
(Handler) Registered new logger logger2 m_data = mydd
(Handler) Registered new logger logger3 m_data = mydd