vehicles: remove historic field from PoolPassiveIoFileMessage

Motivation:
PoolPassiveIoFileMessage contains an array of InetSocketAddress to
tell doors where all possible endpoint for a client and a single
InetSocketAddress, that points only to the first element of the array.
This historic construction makes no sense any more as all dcache around
already handle the array based version.

Modification:
remove remove historic field from PoolPassiveIoFileMessage. Update dcap
door to use PoolPassiveIoFileMessage#socketAddresses.

Result:
less code.

This change is backward compatible, e.g. new doors will work with old
pools, but required new DCAP doors to work with new pools, which is OK.

Acked-by: Paul Millar
Target: master
Require-book: no
Require-notes: yes

modules/dcache-dcap/src/main/java/diskCacheV111/doors/DCapDoorInterpreterV3.java

@@ -2226,7 +2226,7 @@ private void storeChecksumInPnfs( PnfsId pnfsId , String checksumString){
 
         public void poolPassiveIoFileMessage( PoolPassiveIoFileMessage<byte[]> reply) {
 
-            InetSocketAddress poolSocketAddress = reply.socketAddress();
+            InetSocketAddress poolSocketAddress = reply.socketAddresses()[0];
 
             StringBuilder sb = new StringBuilder() ;
             sb.append(_sessionId).append(" ").

modules/dcache-vehicles/src/main/java/diskCacheV111/vehicles/PoolPassiveIoFileMessage.java

@@ -3,10 +3,11 @@
 import java.io.Serializable;
 import java.net.InetSocketAddress;
 
+import static java.util.Objects.requireNonNull;
+
 public class PoolPassiveIoFileMessage<T extends Serializable> extends PoolMessage {
 
     private static final long serialVersionUID = -8019787998659861618L;
-    private final InetSocketAddress _socketAddress;
     private final InetSocketAddress[] _socketAddresses;
     private final T _challange;
 
@@ -30,22 +31,13 @@ public PoolPassiveIoFileMessage(String pool, InetSocketAddress socketAddress, T
      */
     public PoolPassiveIoFileMessage(String pool, InetSocketAddress[] socketAddresses, T challenge,  long verifier) {
         super(pool);
-        _socketAddresses = socketAddresses;
-        _socketAddress = _socketAddresses[0];
+        _socketAddresses = requireNonNull(socketAddresses, "Socket address is not defined.");
         _challange = challenge;
         _verifier = verifier;
     }
 
-    public InetSocketAddress socketAddress() {
-        return _socketAddress;
-    }
-
     public InetSocketAddress[] socketAddresses() {
-        /*
-         * A bit of crappy code to handle old/new version of the class.
-         */
-        return _socketAddresses == null?
-                new InetSocketAddress[] { _socketAddress } : _socketAddresses;
+        return _socketAddresses;
     }
 
     public long getVerifier() {