Create -loglevel configuration option

- add a -loglevel=<level>|<category:level> config option to allow users
  to set a global -loglevel and category-specific log levels. LogPrintLevel
  messages with a higher severity level than -loglevel will not be printed
  in the debug log.

- for now, this config option is debug-only during the migration to
  severity-based logging

- update unit and functional tests

Co-authored-by: "Jon Atack <jon@atack.com>"

src/logging.cpp

@@ -199,7 +199,7 @@ bool GetLogCategory(BCLog::LogFlags& flag, const std::string& str)
     return false;
 }
 
-std::string LogLevelToStr(BCLog::Level level)
+std::string BCLog::Logger::LogLevelToStr(BCLog::Level level) const
 {
     switch (level) {
     case BCLog::Level::Debug: