Refactor WebSessionStore

- Add WebSessionStore.createWebSession.

- Remove remove WebSessionStore.changeSessionId

- Add WebSessionStore updateLastAccessTime which allows updating the
WebSession lastAccessTime without exposing a method on WebSession in
an implementation independent way.

- Remove WebSessionStore.storeSession. This method is not necessary
since the WebSession that is returned allows saving the WebSession.
Additionally, it is error prone since the wrong type might be passed
into it.

Issue: SPR-15875, 15876

spring-web/src/test/java/org/springframework/web/server/session/DefaultWebSessionManagerTests.java

@@ -74,6 +74,9 @@ public class DefaultWebSessionManagerTests {
 
 	@Before
 	public void setUp() throws Exception {
+		when(this.store.createWebSession()).thenReturn(Mono.just(createDefaultWebSession()));
+		when(this.store.updateLastAccessTime(any())).thenAnswer( invocation -> Mono.just(invocation.getArgument(0)));
+
 		this.manager = new DefaultWebSessionManager();
 		this.manager.setSessionIdResolver(this.idResolver);
 		this.manager.setSessionStore(this.store);
@@ -106,6 +109,7 @@ public class DefaultWebSessionManagerTests {
 		session.save().block();
 
 		String id = session.getId();
+		verify(this.store).createWebSession();
 		verify(this.store).storeSession(any());
 		verify(this.idResolver).setSessionId(any(), eq(id));
 	}
@@ -118,6 +122,7 @@ public class DefaultWebSessionManagerTests {
 		session.getAttributes().put("foo", "bar");
 		session.save().block();
 
+		verify(this.store).createWebSession();
 		verify(this.idResolver).setSessionId(any(), any());
 		verify(this.store).storeSession(any());
 	}

spring-web/src/test/java/org/springframework/web/server/session/WebSessionIntegrationTests.java

@@ -115,7 +115,7 @@ public class WebSessionIntegrationTests extends AbstractHttpHandlerIntegrationTe
 		assertNotNull(session);
 		Instant lastAccessTime = Clock.offset(this.sessionManager.getClock(), Duration.ofMinutes(-31)).instant();
 		session = new DefaultWebSession(session, lastAccessTime);
-		store.storeSession(session);
+		session.save().block();
 
 		// Third request: expired session, new session created
 		request = RequestEntity.get(createUri()).header("Cookie", "SESSION=" + id).build();