io_context should process all posted works before shutting down (#865)

* Removed the stop statement while shutting down io_context.

* Added a new test `invocation_should_not_block_indefinitely_during_client_shutdown` which can be run indefinitely and observe that it is not being blocked in any run. An example way to reproduce the problem without this PR fix is :
```
for ((;;)) { /Users/ihsan/Desktop/work/src/hazelcast-cpp-client/cmake-build-arm-debug/hazelcast/test/src/client_test --gtest_filter=IssueTest.invocation_should_not_block_indefinitely_during_client_shutdown ; }
```

This run blocks after several runs in master branch but runs indefinite time with this PR fix with no problem.

Author: ihsandemir

hazelcast/src/hazelcast/client/network.cpp

@@ -165,10 +165,8 @@ namespace hazelcast {
 
                 spi::impl::ClientExecutionServiceImpl::shutdown_thread_pool(executor_.get());
 
+                // release the guard so that the io thread can stop gracefully
                 io_guard_.reset();
-                io_context_->stop();
-                boost::asio::use_service<boost::asio::detail::resolver_service<boost::asio::ip::tcp>>(
-                        *io_context_).shutdown();
                 io_thread_.join();
 
                 connection_listeners_.clear();

hazelcast/test/src/HazelcastTests8.cpp

@@ -1438,6 +1438,35 @@ namespace hazelcast {
                 ASSERT_THROW((map->get<int, int>(1).get()), exception::hazelcast_client_not_active);
             }
 
+            TEST_F(IssueTest, invocation_should_not_block_indefinitely_during_client_shutdown) {
+                HazelcastServer server(*g_srvFactory);
+                auto hz = new_client().get();
+                auto map = hz.get_map("my_map").get();
+
+                ASSERT_NO_THROW(map->put(1, 5).get());
+
+                std::thread t([&] () {
+                   hz.shutdown();
+                });
+
+                try {
+                    int i = 0;
+                    while (true) {
+                        map->put(++i, 5).get();
+                    }
+                } catch (std::exception &) {
+                    // ignore
+                }
+
+                try {
+                    map->put(10, 5).get();
+                } catch (std::exception &) {
+                    // ignore
+                }
+
+                t.join();
+            }
+
             TEST_F(IssueTest, testIssue753) {
                 HazelcastServer server(*g_srvFactory);