Add PBD::QPC::initialize to initialize timer and call it from PBD::init

Check timer for invalid frequency
Precalculate timer tick rate to save a few instructions
Don't use static variables inside functions to avoid checking for initialization
Use static functions inside anonymous namespace for internal linkage

libs/pbd/pbd.cc

@@ -109,6 +109,7 @@ PBD::init ()
 		return false;
 	}
 
+	QPC::initialize();
 	test_timers_from_env ();
 
 	if (!PBD::MMCSS::initialize()) {