Add CompletionStage support in TarantoolClient

Minimal java version changed from 1.6 to 1.8.

Add support of composable operations as ability to return
TarantoolClientOps with CompletionStage as Result. This feature is taken
from peterservice's fork (see
https://github.com/peterservice-rnd/tarantool-java)

FutureImpl is replaced with CompletableFuture under the hood of
TarantoolClientImpl. Note about behaviour of CompletableFuture
(relatively FutureImpl): a result of a CompletableFuture future will not
change after the first call of the 'complete' method (unlike
FutureImpl.setValue).

test: add ClientComposableAsyncOpsIT and AbstractAsyncClientOperationsIT
as ancestor of it and AsyncClientOperationsIT.

Fixes #102

Author: pda4code

Diff for: README.md

@@ -110,17 +110,18 @@ TarantoolClient client = new TarantoolClientImpl(socketChannelProvider, config);
 >   resolution, you could create a function that returns necessary name-to-ID
 >   mappings.
 
-`TarantoolClient` provides three interfaces to execute queries:
+`TarantoolClient` provides four interfaces to execute queries:
 
 * `SyncOps` - returns the operation result
 * `AsyncOps` - returns the operation result as a `Future`
+* `ComposableAsyncOps` - return the operation result as a `CompletionStage`
 * `FireAndForgetOps` - returns the query ID
 
 Feel free to override any method of `TarantoolClientImpl`. For example, to hook
 all the results, you could override this:
 
 ```java
-protected void complete(long code, FutureImpl<?> q);
+protected void complete(long code, CompletableFuture<?> q);
 ```
 
 ## Spring NamedParameterJdbcTemplate usage example

Diff for: pom.xml

@@ -42,8 +42,8 @@
                 <artifactId>maven-compiler-plugin</artifactId>
                 <version>3.2</version>
                 <configuration>
-                    <source>1.6</source>
-                    <target>1.6</target>
+                    <source>1.8</source>
+                    <target>1.8</target>
                 </configuration>
             </plugin>
             <!--

Diff for: src/it/java/org/tarantool/TestTarantoolClient.java

@@ -6,7 +6,7 @@
 import java.nio.channels.SocketChannel;
 import java.sql.SQLException;
 import java.util.Arrays;
-import java.util.List;
+import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.ExecutionException;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Executors;
@@ -81,7 +81,7 @@ protected void reconnect(int retry, Throwable lastError) {
         }
 
         @Override
-        protected void complete(long code, FutureImpl<?> q) {
+        protected void complete(long code, CompletableFuture<?> q) {
             super.complete(code, q);
             if (code != 0) {
                 System.out.println(code);

Diff for: src/main/java/org/tarantool/FutureImpl.java

@@ -2,6 +2,7 @@
 
 import java.util.List;
 import java.util.Map;
+import java.util.concurrent.CompletionStage;
 import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 
@@ -10,9 +11,11 @@ public interface TarantoolClient {
 
     TarantoolClientOps<Integer, List<?>, Object, Future<List<?>>> asyncOps();
 
+    TarantoolClientOps<Integer, List<?>, Object, CompletionStage<List<?>>> composableAsyncOps();
+
     TarantoolClientOps<Integer, List<?>, Object, Long> fireAndForgetOps();
 
-    TarantoolSQLOps<Object, Long, List<Map<String,Object>>> sqlSyncOps();
+    TarantoolSQLOps<Object, Long, List<Map<String, Object>>> sqlSyncOps();
 
     TarantoolSQLOps<Object, Future<Long>, Future<List<Map<String, Object>>>> sqlAsyncOps();