Updated plugin logging.

- The plugin now keeps the log file open for the duration of its lifetime.
 - This greatly improves the performace of the logging component, which was previously a significant bottleneck even at low logging verbosity.
 - The log now prints timestamps to the millisecond.

xpcPlugin/XPCPlugin.cpp

@@ -119,6 +119,7 @@ PLUGIN_API void	XPluginStop(void)
 {
 	XPLMUnregisterFlightLoopCallback(XPCFlightLoopCallback, NULL);
 	XPC::Log::WriteLine("[EXEC] Plugin Shutdown");
+	XPC::Log::Close();
 }
 
 PLUGIN_API void XPluginDisable(void)