Change aging algorithm.

Aging happens when the sum of rankings passes a threshold. Raise this
threshold from 1000 to 6000.
Change aging multiplier from .9 to .99.

This formula is still pretty naive, and wants improvement, but seems to
be nicer. The previous constants tended to forget things too fast.

Closes #53, for now.

z.sh

@@ -62,8 +62,8 @@ _z() {
     count += $2
    }
    END {
-    if( count > 1000 ) {
-     for( i in rank ) print i "|" 0.9*rank[i] "|" time[i] # aging
+    if( count > 6000 ) {
+     for( i in rank ) print i "|" 0.99*rank[i] "|" time[i] # aging
     } else for( i in rank ) print i "|" rank[i] "|" time[i]
    }
   ' 2>/dev/null >| "$tempfile"