JAVA-3055: Prevent PreparedStatement cache to be polluted if a request is cancelled.

There was a critical issue when the external code cancels a request, indeed the cached CompletableFuture will then always throw a CancellationException.
This may happens, for example, when used by reactive like Mono.zip or Mono.firstWithValue.

patch by Luc Boutier; reviewed by Alexandre Dutra and Bret McGuire
reference: #1757

Author: lucboutier

core/src/main/java/com/datastax/oss/driver/internal/core/cql/CqlPrepareAsyncProcessor.java

@@ -159,7 +159,9 @@ public CompletionStage<PreparedStatement> process(
                   });
         }
       }
-      return result;
+      // Return a defensive copy. So if a client cancels its request, the cache won't be impacted
+      // nor a potential concurrent request.
+      return result.thenApply(x -> x); // copy() is available only since Java 9
     } catch (ExecutionException e) {
       return CompletableFutures.failedFuture(e.getCause());
     }