Closes gh-22325

Issue: SPR-17324

Sample output at TRACE:
```
DispatcherServlet - Initializing Servlet 'org.springframework.web.servlet.DispatcherServlet-7a8c8dcf'
AnnotationConfigWebApplicationContext - Refreshing WebApplicationContext for namespace 'org.springframework.web.servlet.DispatcherServlet-7a8c8dcf-servlet', started on Wed Jul 25 17:46:38 EDT 2018
AnnotationConfigWebApplicationContext - Registering [org.springframework.web.servlet.mvc.method.annotation.RequestPartIntegrationTests$CommonsMultipartResolverTestConfig]
AnnotationConfigWebApplicationContext - No 'messageSource' bean, using [Empty MessageSource]
AnnotationConfigWebApplicationContext - No 'applicationEventMulticaster' bean, using [SimpleApplicationEventMulticaster]
AnnotationConfigWebApplicationContext - No 'lifecycleProcessor' bean, using [DefaultLifecycleProcessor]
...
DispatcherServlet - Initialization completed in 3361 ms
```

Issue: SPR-16946

Introduces getBeanProvider(Class) and getBeanProvider(ResolvableType), also narrowing getBean(String, Class) and isTypeMatch(String, Class) to a non-null Class argument and enriching NoUniqueBeanDefinitionException with a full ResolvableType. In addition, ObjectProvider supports iterable/stream access for collection-style resolution of multiple matching beans now, and collection injection falls back to an empty collection in a single-constructor case with non-null arguments.

Issue: SPR-17075
Issue: SPR-11419
Issue: SPR-15338

Issue: SPR-16946

Fix checkstyle violations for javadoc.

Issue: SPR-16968

Issue: SPR-16675

AbstractApplicationContext silently ignores non-initialized ApplicationEventMulticaster/LifecycleProcessor on destruction

Issue: SPR-16149

Issue: SPR-15869

Issue: SPR-15154

This commits extends nullability declarations to the field level, formalizing the interaction between methods and their underlying fields and therefore avoiding any nullability mismatch.

Issue: SPR-15720

Beyond just formally declaring the current behavior, this revision actually enforces non-null behavior in selected signatures now, not tolerating null values anymore when not explicitly documented. It also changes some utility methods with historic null-in/null-out tolerance towards enforced non-null return values, making them a proper citizen in non-null assignments.

Some issues are left as to-do: in particular a thorough revision of spring-test, and a few tests with unclear failures (ignored as "TODO: NULLABLE") to be sorted out in a follow-up commit.

Issue: SPR-15540

Issue: SPR-15540

Issue: SPR-15540

Alignment with HttpServletBean; consistent use of EnvironmentCapable.

Issue: SPR-15469

Issue: SPR-15154

Includes streamlined ClassPathBeanDefinitionScanner setup.

Issue: SPR-14654

Issue: SPR-14783

Issue: SPR-13188

Issue: SPR-10856

Issue: SPR-14140

Previously, the generic type of a simple pojo event implementing
ResolvableTypeProvider wasn't detected properly. This commit fixes the
logic when the generic type is not provided to reuse what
PayloadApplicationEvent is already doing anyway.

Issue: SPR-14029

Issue: SPR-12857
Issue: SPR-13905

Issue: SPR-13783

Issue: SPR-13667

Issue: SPR-13663

Issue: SPR-13556

Issue: SPR-13425

Issue: SPR-13093

AbstractApplicationContext collects early ApplicationEvents and publishes them once the multicaster is available

Issue: SPR-12902

Issue: SPR-12147

Issue: SPR-12147

Add support for annotation-based event listeners. Enabled automatically
when using Java configuration or can be enabled explicitly via the
regular <context:annotation-driven/> XML element. Detect methods of
managed beans annotated with @EventListener, either directly or through
a meta-annotation.

Annotated methods must define the event type they listen to as a single
parameter argument. Events are automatically filtered out according to
the method signature. When additional runtime filtering is required, one
can specify the `condition` attribute of the annotation that defines a
SpEL expression that should match to actually invoke the method for a
particular event. The root context exposes the actual `event`
(`#root.event`) and method arguments (`#root.args`). Individual method
arguments are also exposed via either the `a` or `p` alias (`#a0` refers
to the first method argument). Finally, methods arguments are exposed via
their names if that information can be discovered.

Events can be either an ApplicationEvent or any arbitrary payload. Such
payload is wrapped automatically in a PayloadApplicationEvent and managed
explicitly internally. As a result, users can now publish and listen
for arbitrary objects.

If an annotated method has a return value, an non null result is actually
published as a new event, something like:

@EventListener
public FooEvent handle(BarEvent event) { ... }

Events can be handled in an aynchronous manner by adding `@Async` to the
event method declaration and enabling such infrastructure. Events can
also be ordered by adding an `@order` annotation to the event method.

Issue: SPR-11622