[core] Setting up fmt with custom config structure for logging system

[ethouris]

This provides the helper class fmt_sender_proxy with a helper function fmt so that nondefault formatting can be used in place. All intermediate formatting is done using std::ostringstream or std::stringstream class.

This proposes a new API for all iostream-based solutions, just like also iostream itself. For example: In order to write values of a, x, b in the form like: 17-0001dead-42 using printf you need:

printf("%d-%08x-%d", a, x, b);

And in the standard iostream, you'd have to do the following:

cout << a << "-" << hex << setfill('0') << setw(8) << x << setw(0) << "-" << dec << b;

With this ofmt facility the same can be written simply as:

cout << a << "-" << fmt(x, fmtc().fillzero().hex().width(8)) << "-" << b;

This above can't be used directly without overloading the operator << with fmt_sender_proxy, but this is here available for the LOGC macro. The helper class ofmtstream was provided, which is mainly a wrapper around std::stringstream, but it makes all string-like types bypass the formatting facilities, while all other types (mainly numeric) are passed through fmt with default formatting. Formatting is done by another std::stringstream inside fmt_sender_proxy, which is then copied buffer-to-buffer to the std::stringstream in the ofmtstream class. The operator<< is still in use here for C++03-based code, and for C++11-based code there's additionally the print function, which does the same:

ostr.print(a, "-", fmt(x, fmtc().fillzero().hex().width(8)), "-", b);

The fmtc structure was used because it minimizes scope of the manipulator names, as well as allows to create a variable with format configuration to be then applied in multiple following calls of fmt. Symbol names introduced here:

• fmt: forced formatting function
• fmtc: format configuration structure
• ofmtstream: format handler for multiple arguments
• fmtcat: formats all arguments one by one and returns everything as std::string

TO CONSIDER: Maybe ofmtbufstream would be a better name for this one so that ofmtstream can be used to wrap any std::ostream.

Beside this solution, there were two others so far tried:

1. The sfmt formatter that uses snprintf function and crafts the format string out of the formatter structure - [core] New formatting implementation using SFMT for the logging system #2955
2. The solution based on the {fmt} library that reuses its formatter structure and printing functions (note: requires C++11). - [core] Implementation of logging/formatting system using {fmt} #2963

This solution, out of all that were tried, shows the lowest CPU usage.