Improved logging abstraction

This also gets rid of --log-type, since the nested log type isn't
useful in a multi-threaded situation, and nobody cares about the
"pretty" log type.

src/libstore/globals.hh

@@ -1,6 +1,7 @@
 #pragma once
 
 #include "types.hh"
+#include "logging.hh"
 
 #include <map>
 #include <sys/types.h>