Refactor use of sinks in WebSocketGraphQlTransport

Replace use of Sinks which does not deal with concurrent sending and
use MonoSink and FluxSink instead.

Closes gh-388

Author: rstoyanchev

samples/webflux-security/src/test/java/io/spring/sample/graphql/WebFluxSecuritySampleTests.java

@@ -22,10 +22,9 @@
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
 
-import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.boot.test.context.SpringBootTest;
 import org.springframework.boot.test.context.SpringBootTest.WebEnvironment;
-import org.springframework.boot.web.server.LocalServerPort;
+import  org.springframework.boot.test.web.server.LocalServerPort;
 import org.springframework.graphql.execution.ErrorType;
 import org.springframework.graphql.test.tester.WebGraphQlTester;
 import org.springframework.graphql.test.tester.WebSocketGraphQlTester;

samples/webflux-websocket/src/test/java/io/spring/sample/graphql/WebFluxWebSocketSampleIntegrationTests.java

@@ -24,7 +24,7 @@
 
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.test.context.SpringBootTest;
-import org.springframework.boot.web.server.LocalServerPort;
+import  org.springframework.boot.test.web.server.LocalServerPort;
 import org.springframework.graphql.test.tester.GraphQlTester;
 import org.springframework.graphql.test.tester.WebSocketGraphQlTester;
 import org.springframework.web.reactive.socket.client.ReactorNettyWebSocketClient;