Use a different separator in `Wid`

`#` is a reserved character in the URL spec and must be escaped to be used. This change uses `-` instead.

Fixes: elastic/elasticsearch#467

Original commit: elastic/x-pack-elasticsearch@a75eb89cb8

src/main/java/org/elasticsearch/watcher/execution/Wid.java

@@ -23,7 +23,7 @@ public class Wid {
 
     public Wid(String watchId, long nonce, DateTime executionTime) {
         this.watchId = watchId;
-        this.value = watchId + "_" + String.valueOf(nonce) + "#" +  formatter.print(executionTime);
+        this.value = watchId + "_" + String.valueOf(nonce) + "-" +  formatter.print(executionTime);
     }
 
     public Wid(String value) {