Prevent reusing cached cancelled requests

There was a critical issue in the driver that may occur when the external code cancels a request, indeed the CompletableFuture will then always throw a CancellationException.

This could occur for example when used in a reactive context with a Mono.zip failed.

Author: lucboutier

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

@@ -142,7 +142,10 @@ public CompletionStage<PreparedStatement> process(
 
     try {
       CompletableFuture<PreparedStatement> result = cache.getIfPresent(request);
-      if (result == null) {
+      if (result == null || result.isCancelled()) {
+        if (result.isCancelled()) {
+          cache.invalidate(request);
+        }
         CompletableFuture<PreparedStatement> mine = new CompletableFuture<>();
         result = cache.get(request, () -> mine);
         if (result == mine) {