Issue #2372389 by znerol, almaudoh: Expose session handler in container

core/core.services.yml

core/lib/Drupal/Core/CoreServiceProvider.php

...ependencyInjection/Compiler/StackedSessionHandlerPass.php

core/lib/Drupal/Core/Session/SessionManager.php

core/lib/Drupal/Core/Session/SessionManagerInterface.php

.../src/Tests/Session/StackSessionHandlerIntegrationTest.php

...ystem/tests/modules/session_test/session_test.routing.yml

...stem/tests/modules/session_test/session_test.services.yml

...les/session_test/src/Controller/SessionTestController.php

...ules/session_test/src/Session/TestSessionHandlerProxy.php