so-5.5.0: An error_logger interface

Previous versions of SObjectizer use ACE_Logger facility from ACE Framework. In the case of an error SObjectizer core used one of ACE_Logger macros (e.g. ACE_DEBUG) to place error message to appropriate error stream or/and log file. If user want to store error messages to some specific destination he/she had to tune ACE_Logger (at the start of application for example).

But since v.5.5.0 SObjectizer doesn't use ACE Framework any more. So if user wants to store SObjectizer's error messages to some location he/she must define and specify its own error_logger to the SObjectizer Environment parameters. Something like this:

class custom_error_logger : public so_5::rt::error_logger_t
{
public :
    // Constructor.
    custom_error_logger( some_log_interface log ) : log_(log) {}

    // Error message handler.
    virtual void log(
        const char * file,
        unsigned int line,
        const std::string & message ) override
    {
        log_.store_message( message );
    }

private :
    some_log_interface log_;
};
...
// Tuning SObjectizer Environment parameters at start.
some_log_interface log = create_log(...);
...
so_5::launch(
    []( so_5::rt::environment_t & env )
    {
        // Some initialization stuff...
    },
    [log]( so_5::rt::environment_params_t & params )
    {
        // Setting logger.
        params.error_logger( so_5::rt::error_logger_shptr_t(
            new custom_error_log( log ) ) );
        ...
    } );

If custom error logger is not specified by the user then a default error logger will be used. SObjectizer creates default error logger by so_5::create_stderr_logger(). This logger uses std::cerr as error stream.