Extract interface Availability (#191)

* Extract interface `AvailabilityProvider`

#### Problem

`DuplexConnection`, `ReactiveSocket` and `ReactiveSocketClient` all provide a method `double availability()` and so can benefit from having a common interface.
This will be useful for giving event callbacks that retrieve availability as availability follows a pull model instead of push.

#### Modification

Added a new interface `AvailabilityProvider` and have the other interfaces extend it.

#### Result

Common way to access availabilty.

* Rename `AvailabilityProvider` to `Availability`

Author: NiteshKant

reactivesocket-core/src/main/java/io/reactivesocket/Availability.java

@@ -0,0 +1,24 @@
+/*
+ * Copyright 2016 Netflix, Inc.
+ * <p>
+ *  Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with
+ *  the License. You may obtain a copy of the License at
+ *  <p>
+ *  http://www.apache.org/licenses/LICENSE-2.0
+ *  <p>
+ *  Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on
+ *  an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the
+ *  specific language governing permissions and limitations under the License.
+ */
+
+package io.reactivesocket;
+
+public interface Availability {
+
+    /**
+     * @return a positive numbers representing the availability of the entity.
+     * Higher is better, 0.0 means not available
+     */
+    double availability();
+
+}

reactivesocket-core/src/main/java/io/reactivesocket/DuplexConnection.java

@@ -24,7 +24,7 @@
 /**
  * Represents a connection with input/output that the protocol uses. 
  */
-public interface DuplexConnection {
+public interface DuplexConnection extends Availability {
 
     /**
      * Sends the source of {@link Frame}s on this connection and returns the {@code Publisher} representing the result
@@ -77,12 +77,6 @@ default Publisher<Void> sendOne(Frame frame) {
      */
     Publisher<Frame> receive();
 
-    /**
-     * @return the availability of the underlying connection, a number in [0.0, 1.0]
-     * (higher is better).
-     */
-    double availability();
-
     /**
      * Close this {@code DuplexConnection} upon subscribing to the returned {@code Publisher}
      *

reactivesocket-core/src/main/java/io/reactivesocket/ReactiveSocket.java

@@ -17,12 +17,11 @@
 package io.reactivesocket;
 
 import org.reactivestreams.Publisher;
-import io.reactivesocket.reactivestreams.extensions.Px;
 
 /**
  * A contract providing different interaction models for <a href="https://github.com/ReactiveSocket/reactivesocket/blob/master/Protocol.md">ReactiveSocket protocol</a>.
  */
-public interface ReactiveSocket {
+public interface ReactiveSocket extends Availability {
 
     /**
      * Fire and Forget interaction model of {@code ReactiveSocket}.
@@ -71,11 +70,7 @@ public interface ReactiveSocket {
      */
     Publisher<Void> metadataPush(Payload payload);
 
-    /**
-     * Client check for availability to send request based on lease
-     *
-     * @return 0.0 to 1.0 indicating availability of sending requests
-     */
+    @Override
     default double availability() {
         return 0.0;
     }

reactivesocket-core/src/main/java/io/reactivesocket/client/ReactiveSocketClient.java

@@ -17,15 +17,14 @@
 package io.reactivesocket.client;
 
 import io.reactivesocket.AbstractReactiveSocket;
+import io.reactivesocket.Availability;
 import io.reactivesocket.ReactiveSocket;
 import io.reactivesocket.lease.DisabledLeaseAcceptingSocket;
 import io.reactivesocket.lease.LeaseEnforcingSocket;
 import io.reactivesocket.transport.TransportClient;
 import org.reactivestreams.Publisher;
 
-import java.util.function.Function;
-
-public interface ReactiveSocketClient {
+public interface ReactiveSocketClient extends Availability {
 
     /**
      * Creates a new {@code ReactiveSocket} every time the returned {@code Publisher} is subscribed.
@@ -34,12 +33,6 @@ public interface ReactiveSocketClient {
      */
     Publisher<? extends ReactiveSocket> connect();
 
-    /**
-     * @return a positive numbers representing the availability of the factory.
-     * Higher is better, 0.0 means not available
-     */
-    double availability();
-
     /**
      * Creates a new instances of {@code ReactiveSocketClient} using the passed {@code transportClient}. This client
      * will not accept any requests from the server, so the client is half duplex. To create full duplex clients use