(Accord) AsyncChain.flatMap does not begin the result of the flatMap, which causes AsyncLoader to hang in some cases (#81)

patch by David Capwell; reviewed by Benedict Elliott Smith, David Capwell for CASSANDRA-19405
diff --git a/accord-core/src/main/java/accord/utils/async/AsyncChains.java b/accord-core/src/main/java/accord/utils/async/AsyncChains.java
index fcc2956..579bbf9 100644
--- a/accord-core/src/main/java/accord/utils/async/AsyncChains.java
+++ b/accord-core/src/main/java/accord/utils/async/AsyncChains.java
@@ -570,7 +570,7 @@
                     executor.execute(() -> {
                         try
                         {
-                            mapper.apply(v).addCallback(callback);
+                            mapper.apply(v).begin(callback);
                         }
                         catch (Throwable t)
                         {