Issue: SPR-17086

Issue: SPR-17486

Issue: SPR-17486

Issue: SPR-17482

Issue: SPR-17473

In particular between reactive and non-reactive web variants, but
also preparing for a messaing reactive variant.

This commit makes the 3 existing InvocableHandlerMethod types more
consistent and comparable with each other.

1. Use of consistent method names and method order.

2. Consistent error formatting.

3. Explicit for loops for resolving argument values in webflux variant
because that makes it more readable, creates less garabage, and it's
the only way to bring consistency since the other two variants cannot
throw exceptions inside Optional lambdas (vs webflux variant which can
wrap it in a Mono).

4. Use package private HandlerMethodArgumentComposite in webflux
variant in order to pick up the resolver argument caching that the
other two variants have.

5. Polish tests.

6. Add missing tests for messaging variant.

Prior to this commit, no `ResourceUrlProvider` was configured
in WebFlux (no bean was contributed by the WebFlux infrastructure).
Also, several `ResourceTransformer` instances that extend the
`ResourceTransformerSupport` base class need a `ResourceUrlProvider`
to resolve absolute URLs when rewriting resource URLs. At this point,
no `ResourceUrlProvider` was configured and they could only resolve
relative URLs.

This commit contributes a new `ResourceUrlProvider` to the WebFlux
configuration; this bean can be reused by the WebFlux infrastructure and
application code.

This also automatically configure this shared `ResourceUrlProvider`
instance on the resource chain where needed.

Issue: SPR-17433

Prior to this commit, `ResourceTransformerSupport.toAbsolutePath`
would call `StringUtils.applyRelativePath` in all cases. But this
implementation is prepending the given path even if the relative path
starts with `"/"`.

This commit skips the entire operation if the given path is absolute,
i.e. it starts with `"/"`.

Issue: SPR-17432

Issue: SPR-17420

Review and update Servlet and Undertow adapters to release any data
buffers they be holding on to at the time of error or cancellation.

Also remove onDiscard hooks from Reactor and Undertow request body.
For Reactor we expect it to be handled. For Undertow there isn't
any Reactor Core upstream for the callback to be useful.

Issue: SPR-17410

This commit changes the attributes stored under
RouterFunctions.MATCHING_PATTERN_ATTRIBUTE and
HandlerMapping.BEST_MATCHING_PATTERN_ATTRIBUTE from a String to a
PathPattern, similar to what annotated controllers set.

Issue: SPR-17395

SPR-17367

Several benchmarks underlined a few hotspots for CPU and GC pressure in
the Spring Framework codebase:

1. `org.springframework.util.MimeType.<init>(String, String, Map)`
2. `org.springframework.util.LinkedCaseInsensitiveMap.convertKey(String)`

Both are linked with HTTP request headers parsing and response headers
writin during the exchange processing phase.

1) is linked to repeated calls to `HttpHeaders.getContentType`
within a single request handling. The media type parsing operation
is expensive and the result doesn't change between calls, since
the request headers are immutable at that point.

This commit improves this by caching the parsed `MediaType` for the
`"Content-Type"` request header in the `ReadOnlyHttpHeaders` class.
This change is available for both Spring MVC and Spring WebFlux.

2) is linked to insertions/lookups in the `LinkedCaseInsensitiveMap`,
which is the data structure behind `HttpHeaders`.
Those operations are creating a lot of garbage (including a lot of
`String` created by `toLowerCase`). We could choose a more efficient
data structure for storing HTTP headers data.

As a first step, this commit is focusing on Spring WebFlux and
introduces `MultiValueMap` implementations mapped by native HTTP headers
for the following servers: Tomcat, Jetty, Netty and Undertow.
Such implementations avoid unnecessary copying of the headers
and leverages as much as possible optimized operations provided by the
native implementations.

This change has a few consequences:

* `HttpHeaders` can now wrap a `MultiValueMap` directly
* The default constructor of `HttpHeaders` is still backed by a
`LinkedCaseInsensitiveMap`
* The HTTP request headers for the websocket HTTP handshake now need to
be cloned, because native headers are likely to be pooled/recycled by
the server implementation, hence gone when the initial HTTP exchange is
done

Issue: SPR-17250

Issue: SPR-17326

Issue: SPR-17338

With this commit, we now make sure that the attributes are not cleared
if they are same as the nested request.

Issue: SPR-17304

In order to be able to leverage WebFlux configuration in a functional
way, WebHttpHandlerBuilder and RouterFunctionMapping should leverage
new ObjectProvider capabilities to get a sorted list of beans by type
instead of using autowired containers.

Issue: SPR-17327

Switch to using the sendClose method available since Reactor Netty 0.8
vs explicitly sending a CloseWebSocketFrame.

Related to SPR-17306, but does not address the root cause.

Issue: SPR-17192

Issue: SPR-16228

See: SPR-17054

This commit stores the first matching path pattern in the attribute
`RouterFunctions.MATCHING_PATTERN_ATTRIBUTE`.

Issue: SPR-17098

This commit restores the attributes when either of the predicates in
an AND/OR conjunction is false.

Issue: SPR-17210

1. Helper method to eliminate duplication in formatting (de-)serialized
   values for logging introduced with prior commit #e62298.

2. Helper method for TRACE vs DEBUG logging with different details.

Issue: SPR-17254

This reverts commit 253ffd71.

Since the `initStrategies` method is there to be extended, the related
attributes should be `protected` as well.

Issue: SPR-17174

Issue: SPR-17174

This reverts commit 7e917335.