Synchronise the KVs list for scanner results

Synchronises the list that holds the KeyValues that have been produced
by the scanner callbacks. The list is accessed from multiple threads at
a time and wasn't thread-safe, causing inconsistent results and partial
loss of data in the response.

Relates to: #1753
Resolves: #1760

src/core/SaltScanner.java

@@ -464,7 +464,7 @@ final class ScannerCB implements Callback<Object,
     ArrayList<ArrayList<KeyValue>>> {
     private final Scanner scanner;
     private final int index;
-    private final List<KeyValue> kvs = new ArrayList<KeyValue>();
+    private final List<KeyValue> kvs = Collections.synchronizedList(new ArrayList<KeyValue>());
     private final ByteMap<List<Annotation>> annotations = 
             new ByteMap<List<Annotation>>();
     private final Set<String> skips = Collections.newSetFromMap(