Add global concurrency parameter. fix #56

Fix bug which caused pending actions to be run multiple times when pending time finished
Fix bug which caused
1. past pending actions to rerun on exit
2. maximum one pending action per pattern/action couple to run on exit

config/example.jsonnet

@@ -21,6 +21,11 @@ local iptables(args) = ['ip46tables', '-w'] + args;
     },
   },
 
+  // if set to a positive number → max number of concurrent actions
+  // if set to a negative number → no limit
+  // if not specified or set to 0 → defaults to the number of CPUs on the system
+  concurrency: 0,
+
   // Those commands will be executed in order at start, before everything else
   start: [
     // Create an iptables chain for reaction