JsonRpcConnection: Don't drop client from cache prematurely

PR #7445 incorrectly assumed that a peer that had already disconnected
and never reconnected was due to the endpoint client being dropped after
a successful socket shutdown. Howeever, the issue at that time was that
there was not a single timeout guards that could cancel the `async_shutdown`
call, petentially blocking indefinetely. Although remove the client from
cache early might have allowed the endpoint to reconnect, it did not
resolve the underlying problem. Now that we have a proper cancellation
timeout, we can wait until the currently used socket is fully closed
before dropping the client from our cache. When our socket termination
works reliably, the `ApiListener` reconnect timer should attempt to
reconnect this endpoint after the next tick.

lib/remote/jsonrpcconnection.cpp

@@ -209,17 +209,7 @@ void JsonRpcConnection::Disconnect()
 
 		IoEngine::SpawnCoroutine(m_IoStrand, [this, keepAlive](asio::yield_context yc) {
 			Log(LogWarning, "JsonRpcConnection")
-				<< "API client disconnected for identity '" << m_Identity << "'";
-
-			// We need to unregister the endpoint client as soon as possible not to confuse Icinga 2,
-			// given that Endpoint::GetConnected() is just performing a check that the endpoint's client
-			// cache is not empty, which could result in an already disconnected endpoint never trying to
-			// reconnect again. See #7444.
-			if (m_Endpoint) {
-				m_Endpoint->RemoveClient(this);
-			} else {
-				ApiListener::GetInstance()->RemoveAnonymousClient(this);
-			}
+				<< "Disconnecting API client for identity '" << m_Identity << "'";
 
 			m_OutgoingMessagesQueued.Set();
 
@@ -255,6 +245,15 @@ void JsonRpcConnection::Disconnect()
 			shutdownTimeout->Cancel();
 
 			m_Stream->lowest_layer().shutdown(m_Stream->lowest_layer().shutdown_both, ec);
+
+			if (m_Endpoint) {
+				m_Endpoint->RemoveClient(this);
+			} else {
+				ApiListener::GetInstance()->RemoveAnonymousClient(this);
+			}
+
+			Log(LogWarning, "JsonRpcConnection")
+				<< "API client disconnected for identity '" << m_Identity << "'";
 		});
 	}
 }