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.h

@@ -189,6 +189,9 @@ namespace BCLog {
         //! Returns a string with all user-selectable log levels.
         std::string LogLevelsString() const;
 
+        //! Returns the string representation of a log level.
+        std::string LogLevelToStr(BCLog::Level level) const;
+
         bool DefaultShrinkDebugFile() const;
     };