Improve multithreaded performance (#1)

Mutex lock is not necessary when performing line formatting and conversion to UTF-8, so I have moved string manipulation out of the locked scope. This gave more concurrency level and about 3 times more performance on Windows with my test sample.

include/plog/Appenders/RollingFileAppender.h

@@ -35,6 +35,8 @@ namespace plog
 
         virtual void write(const Record& record) PLOG_OVERRIDE
         {
+            const std::string& line = Converter::convert(Formatter::format(record));
+
             util::MutexLock lock(m_mutex);
 
             if (m_firstWrite)
@@ -47,7 +49,7 @@ namespace plog
                 rollLogFiles();
             }
 
-            size_t bytesWritten = m_file.write(Converter::convert(Formatter::format(record)));
+            size_t bytesWritten = m_file.write(line);
 
             if (static_cast<size_t>(-1) != bytesWritten)
             {