[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 << "-" << dec << b;

(Setting width back to 0 is not necessary because it's done after every printed integer.)

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

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

To use operator << this way, you have to also include ofmt_iostream.h. For the LOGC macro it is already provided.

The helper class ofmtbufstream was provided for any intermediate formatting. It's mainly a wrapper over std::stringstream, while all string-like classes are written directly to the stream (bypass formatting), 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 ofmtbufstream 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);

It was chosen to have fmtc with manipulators implemented as its methods in order to minimize namespace pollution. This allows also to create a variable for a shortcut name of format configuration to be then applied in multiple following calls of fmt. Symbol names introduced here:

• fmt: forced formatting function (returns the value-wrapping proxy to apply formatting when sent to the stream)
• fmts: like fmt, but returns the formatted value as std::string
• fmtc: format configuration structure
• ofmtbufstream: format handler for multiple arguments
• ofmtrefstream: format handler as a wrapper over std::ostream class
• fmtcat: formats all arguments one by one and returns everything as std::string

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.