Switch completely to Context Logger Instance

To ensure passing context in each logger call and thus contribute to log correlation, switch completely to Context Logger across the complete code. Also,

1. Remove all the public log methods in log_classic.go; they force stacktrace walk-through for lookup of right logger instance which degrades performance.
2. Mark Logger instance as Depreciated; can't remove it yet as it is being used by openonu adapter go version.
3. Also introduce a logger instance in log package and use it instead of default logger. It will allow better control of log level of that package.