util: Establish global logger object.

The object encapsulates logging configuration, and in a later commit,
set up routines will also be moved into the class.

src/test/test_bitcoin.cpp

@@ -47,7 +47,7 @@ BasicTestingSetup::BasicTestingSetup(const std::string& chainName)
         SetupNetworking();
         InitSignatureCache();
         InitScriptExecutionCache();
-        fPrintToDebugLog = false; // don't want to write to debug.log file
+        g_logger->fPrintToDebugLog = false; // don't want to write to debug.log file
         fCheckBlockIndex = true;
         SelectParams(chainName);
         noui_connect();