Prefetch pawn hash key

Plus a bunch of other minor optimizations.

With this power pack we have an increase
of a whopping 1.4%  :-)

...and it took 3 good hours of profiling + hacking to get it out !

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>

src/misc.h

@@ -56,6 +56,7 @@ extern int get_system_time();
 extern int cpu_count();
 extern int Bioskey();
 extern void prefetch(char* addr);
+extern void prefetchPawn(Key, int);
 
 
 ////