Do not require BackendKeyData (impossibl#424)

Some databases, such as CockroachDB, do not send BackendKeyData on
connection initialization. Previously, the ServerConnectionFactory would
assume that BackendKeyData would always be present which caused a
NullPointerException. Now, the BackendKeyData defaults to 0 instead of
null.

If the BackendKeyData is not set, then the CancelRequestTask
short-circuits and logs a warning.

driver/src/main/java/com/impossibl/postgres/jdbc/CancelRequestTask.java

@@ -44,11 +44,14 @@
 import java.net.Socket;
 import java.net.SocketAddress;
 import java.nio.ByteBuffer;
+import java.util.logging.Logger;
 
 import io.netty.channel.unix.DomainSocketAddress;
 
 public class CancelRequestTask extends ExecutionTimerTask {
 
+  private static final Logger logger = Logger.getLogger(CancelRequestTask.class.getName());
+
   private SocketAddress serverAddress;
   private ServerConnection.KeyData keyData;
 
@@ -64,6 +67,11 @@ public void go() {
 
   private void sendCancelRequest() {
 
+    if (keyData.getProcessId() == 0 && keyData.getSecretKey() == 0) {
+      logger.warning("Cannot send CancelRequest because of missing BackendKeyData.");
+      return;
+    }
+
     try {
 
       if (serverAddress instanceof InetSocketAddress) {

driver/src/main/java/com/impossibl/postgres/protocol/v30/ServerConnectionFactory.java

@@ -495,7 +495,7 @@ public void handleNegotiate(Version maxProtocolVersion, List<String> unrecognize
       }
 
       @Override
-      public void handleComplete(Integer processId, Integer secretKey, Map<String, String> parameterStatuses, List<Notice> notices) {
+      public void handleComplete(int processId, int secretKey, Map<String, String> parameterStatuses, List<Notice> notices) {
 
         startupParameterStatuses.putAll(parameterStatuses);
         startupKeyData.set(new KeyData(processId, secretKey));

driver/src/main/java/com/impossibl/postgres/protocol/v30/StartupRequest.java

@@ -63,16 +63,16 @@ interface CompletionHandler {
 
     void handleNegotiate(Version maxProtocolVersion, List<String> unrecognizedParameters) throws IOException;
 
-    void handleComplete(Integer processId, Integer secretKey, Map<String, String> parameterStatuses, List<Notice> notices) throws IOException;
+    void handleComplete(int processId, int secretKey, Map<String, String> parameterStatuses, List<Notice> notices) throws IOException;
     void handleError(Throwable cause, List<Notice> notices) throws IOException;
 
   }
 
   private Version protocolVersion;
   private Map<String, Object> startupParameters;
   private CompletionHandler handler;
-  private Integer backendProcessId;
-  private Integer backendSecretKey;
+  private int backendProcessId;
+  private int backendSecretKey;
   private Map<String, String> parameterStatuses;
   private List<Notice> notices;