Improve Signature sorting speed

Changed the signature sorting code to use a a single merge sort instead
of the multiple pass sorting that was being used. This reduces startup
time on Tile by a factor of 3.

Also replace the user array of pointers to ints with a simpler array of
ints.
Fix compiler warning

app-layer-parser.c: In function ‘AppLayerPPTestData’:
app-layer-parser.c:2525:9: error: variable ‘dir’ set but not used [-Werror=unused-but-set-variable]
     int dir = 0;
         ^