Improve logger implementation

[CromFr]

The current logger implementation is very basic: it just appends lines to a txt file.

Features to implement:

• Log file rotation (to prevents huge log files)
• Improve logger configuration:
  • Set file open mode: append (a) or overwrite on startup (w)
  • Set log verbosity (already implemented)
  • Configure log rotation (max size, retention duration)

Ideally there should be a "global log config" in nwnx.ini, that can be overridden by plugin-specific config files

[scottmunday84]

Maybe we're ready for Boost libs? There's a lot of great enhancements available to the library and it works well for Modern C++. It also has a great logging framework with rotation, mode, and verbosity handlers (defer to BOOST our current logic).