Closes gh-1108

 - Added Woodstox before Aalto in the spring-web build, so that Woodstox
   is used as StAX implementation, and not the less featured Aalto.
 - Hardcoded Aalto dependency in XmlEventDecoder, instead of relying on
   the StAX XMLInputFactory.

Issue: SPR-4365

Issue: SPR-14459

Closes gh-1110

Closes gh-1070

This commit fix a potential `ArrayIndexOutOfBoundsException` if
`lazyLoadArguments` is called with an empty variable argument.

See gh-1070

Issue: SPR-14438

Issue: SPR-14457

Issue: SPR-14457

Issue: SPR-14455

Issue: SPR-14431

Prior to this commit, a dynamic test in
FailingBeforeAndAfterMethodsSpringExtensionTestCase was failing but for
the wrong reason. Namely, the @Configuration class was private which
resulted in an IllegalStateException being thrown, when in fact
an AssertionFailedError was expected.

This commit addresses this by introducing an explicit check for an
AssertionFailedError.

Issue: SPR-4365

Prior to this commit, if multiple TestExecutionListener 'after' methods
threw an exception, only the first such exception was rethrown.
Subsequent exceptions were logged, but there was no way to access or
process them other than via the log file.

This commit addresses this shortcoming by making use of the support for
suppressed exceptions introduced in Java 7. Specifically, if multiple
TestExecutionListener 'after' methods throw an exception, the first
exception will be rethrown with subsequent exceptions suppressed in the
first one.

Issue: SPR-14459

Issue: SPR-14452

This commit also aligns ResourceUrlProvider's and RequestMappingInfo's UrlPathHelper setter/getter signatures.

Issue: SPR-14454

Moved ResponseBodyProcessor creation from constructor to
writeWithInternal(), in preparation of supporting both
Publisher<DataBuffer> as well as Publisher<Publisher<DataBuffer>>.

Moved log4j.properties from the main source tree to the test tree.

In preparation of supporting both Publisher<DataBuffer> and
Publisher<Publisher<DataBuffer>> as response body, moved
RequestBodyPublisher and ResponseBodyProcessor into
ServletServerHttpRequest and ServletServerHttpResponse respectively.

Refactored Publisher<Void> in AbstractResponseBodyProcessor into
separate ResponseBodyWriteResultPublisher.

A "What's New" page has been created in the Spring Framework wiki:

https://github.com/spring-projects/spring-framework/wiki/What's-New-in-the-Spring-Framework

This commit therefore links to the new wiki page from the appendix of
the reference manual and deletes the existing "What's New" chapter from
the reference manual.

Issue: SPR-14449

This commit makes use of the new Supplier<String> variants of utility
methods in org.springframework.util.Assert within the spring-test
module.

Issue: SPR-14450

This commit makes use of the new Supplier<String> variants of utility
methods in org.springframework.util.Assert within the spring-core
module.

Issue: SPR-14450

Prior to this commit, utility methods in
org.springframework.util.Assert accepted String arguments for custom
error messages. Such Strings are evaluated (e.g., concatenated)
eagerly, and for performance reasons, it was therefore not always
possible to make use of these utility methods. Consequently, there are
several places in the code base that "inline" identical functionality
in order to lazily evaluate error message concatenation and avoid an
unnecessary performance penalty. This leads to verbose code like the
following.

if (!contextPath.startsWith("/")) {
    throw new IllegalArgumentException("contextPath '" + contextPath +
    "' must start with '/'.");
}
if (contextPath.endsWith("/")) {
    throw new IllegalArgumentException("contextPath '" + contextPath +
    "' must not end with '/'.");
}

This commit addresses this shortcoming by introducing Supplier<String>
variants of all utility methods in org.springframework.util.Assert that
allow custom error messages to be evaluated lazily via a lambda
expression that is only evaluated if the assertion fails. This results
in a simplification of the previous examples as follows.

Assert.isTrue(contextPath.startsWith("/"), () -> "contextPath '" +
    contextPath + "' must start with '/'.");
Assert.isTrue(!contextPath.endsWith("/"), () -> "contextPath '" +
    contextPath + "' must not end with '/'.");

Issue: SPR-14450

Issue: SPR-14247