Get system timestamp less often (only after sleeping/selecting)

src/util/select.h

@@ -39,6 +39,7 @@
 #include <sys/select.h>
 
 #include "fatal_assert.h"
+#include "timestamp.h"
 
 /* Convenience wrapper for pselect(2).
 
@@ -138,6 +139,8 @@ public:
       ret = 0;
     }
 
+    freeze_timestamp();
+
     return ret;
   }