Add logging to ease debugging connection reattempts

[blitzrk]

Background: I was setting up tls+tcp connection with mutual authentication. I didn't realize that I needed to provide a ServerName in the tls.Config, so mangos kept trying to reconnect, but I just saw this as a timeout occurring from my code. I eventually had to throw in print statements throughout core.go to find the cause.

I propose adding a very simple logging solution.

My idea is to let users register loggers (like how database/sql registers drivers). A default one that logs to stderr could be provided in a mangos subpackage. I'll submit a PR with a proof of concept in just a minute. Using it would look like this:

// Not a working example
package mangos_test

import (
        "crypto/tls"
        "crypto/x509"
        "log"

        "github.com/go-mangos/mangos"
        "github.com/go-mangos/mangos/protocol/push"
        "github.com/go-mangos/mangos/transport/tlstcp"

        _ "github.com/go-mangos/mangos/log"
)

var (
        url  = "localhost:5555"
        conf = &tls.Config{
                Certificates: []tls.Certificate{},
                RootCAs:      x509.NewCertPool(),
        }
)

func init() {
        mangos.SetLogLevel(2)
}

func Example() {
        sock, err = push.NewSocket()
        if err != nil {
                log.Fatalf("can't get new push socket: %v", err)
        }
        sock.AddTransport(tlstcp.NewTransport())
        if err = sock.DialOption("tls+tcp://"+url, conf); err != nil {
                log.Fatalf("can't dial on push socket: %v", err)
        }
        defer sock.Close()

        if err = sock.Send([]byte("Hello world!")); err != nil {
                log.Fatalf("can't send message on push socket: %v", err)
        }
}

[blitzrk]

By default, all calls in mangos that use log.{Info,Infof,Debug,Debugf} would be noops. By importing a logger such as with import _ "github.com/go-mangos/mangos/log" a stderr logger would be registered, but all logging would still be a noop until a log level was set. With a call to mangos.SetLogLevel(1), future Info logging would be logged. At log level 2 (or greater), debug info would also be logged.

I only added a single log usage to the main code where it was useful for me, but more could easily be added.

[lobocv]

@gdamore You've mentioned your aversion to adding dependencies to the project (logrus), if that is still the case then may I suggest we define an interface that defines hooks which we can then add to the library.
An example of a hook would be a func(args map[string]interface{}). The user can then define their adapter which could implement the func and translate it into their preferred logger (ie stdlib or logrus).

What do you think?

Example:
https://play.golang.org/p/_r-a-n09GY9

[gdamore]

I'd be fine with a hook style interface.