nfs4: make extended NFSv4StateHandler constructor public

Motivation:
the NFSv4StateHandler(leaseTime, instanceId, clientStore)  constructor
can be used by NFS servers to control NFSv4StateHandler instance
creation. As extended constructor is package protected, thus control is
unavailable to other modules.

Modification:
make extended NFSv4StateHandler constructor public. Updated javadoc.

Result:
more control on how instance of NFSv4StateHandler created.

Acked-by: Paul Millar
Target: master

core/src/main/java/org/dcache/nfs/v4/NFSv4StateHandler.java

@@ -92,7 +92,15 @@ public NFSv4StateHandler() {
         this(NFSv4Defaults.NFS4_LEASE_TIME, 0, new EphemeralClientRecoveryStore());
     }
 
-    NFSv4StateHandler(long leaseTime, int instanceId, ClientRecoveryStore clientStore) {
+    /**
+     * Create NFSv4 state handler with given lease time, instance id and client store.
+     * The {@code instanceId} should uniquely identify this state handler.
+     *
+     * @param leaseTime state lease time in seconds.
+     * @param instanceId the nfs server instance id within deployment.
+     * @param clientStore store used by state handler to keep track of valid clients.
+     */
+    public NFSv4StateHandler(long leaseTime, int instanceId, ClientRecoveryStore clientStore) {
         _leaseTime = TimeUnit.SECONDS.toMillis(leaseTime);
         _clientsByServerId = new Cache<>("NFSv41 clients", 5000, Long.MAX_VALUE,
                 _leaseTime * 2,