[pulsar-client-cpp] Fix memory leak caused by not being executed ClientConnection destructor

[massakam]

Motivation

Currently, closing a C++ Pulsar client object does not execute the destructor for the generated ClientConnection object. As a result, memory usage increases in applications that repeatedly create and close clients.

This pull-request is a continuation of #5246.

Modifications

The reason why the ClientConnection destructor is not executed is that the reference count of the shared pointer does not become 0 even when the ClientConnection object is closed. As a result of the investigation, it was found that if the shared pointer executor_ held by the ClientConnection object is reset, the reference count becomes 0 and the destructor is executed.

--- a/pulsar-client-cpp/lib/ClientConnection.cc
+++ b/pulsar-client-cpp/lib/ClientConnection.cc
@@ -1385,6 +1385,10 @@ void ClientConnection::close() {
     if (tlsSocket_) {
         tlsSocket_->lowest_layer().close();
     }
+
+    if (executor_) {
+        executor_.reset();
+    }
 }

 bool ClientConnection::isClosed() const { return state_ == Disconnected; }

However, with this fix alone, executing the following code causes an "Illegal instruction" error and the program crashes.
https://gist.github.com/massakam/ebd169caff3db802eee5001b0d06c980

The exact cause is unknown, but this issue doesn't seem to occur if closing ClientConnection before closing ExecutorServiceProvider. So, I changed the ConnectionPool destructor added in #5246 to the close() method and call it before closing ExecutorServiceProvider.

[massakam]

rerun cpp tests

[massakam]

rerun java8 tests