FailsafeFuture: clear execution after completion (#362)

This should fix result futures strongly retaining their computing functions

Author: stevenschlansker

core/src/main/java/dev/failsafe/spi/FailsafeFuture.java

@@ -81,6 +81,8 @@ public synchronized boolean cancel(boolean mayInterruptIfRunning) {
     ExecutionResult<R> result = ExecutionResult.exception(new CancellationException());
     super.completeExceptionally(result.getException());
     completionHandler.accept(result, newestExecution);
+    newestExecution = null;
+    cancelFunctions = null;
     return cancelResult;
   }
 
@@ -99,6 +101,8 @@ public synchronized boolean completeResult(ExecutionResult<R> result) {
       completed = super.completeExceptionally(exception);
     if (completed)
       completionHandler.accept(result, newestExecution);
+    newestExecution = null;
+    cancelFunctions = null;
     return completed;
   }