Rename handshakeAttributes to just attributes

Issue: SPR-11566
Add session attributes to SimpMessageHeaderAccessor

This change exposes the WebSocketSession attributes through a message header.
The StompSubProtocolHandler adds this to incoming messages.
For now messaging handling  methods can access the map via @Header, e.g.:

@Header(StompHeaderAccessor.SESSION_ATTRIBUTES) Map<String, Object> attrs) {

Issue: SPR-11566
Support meta-annotation overrides in ASM processing

Prior to this commit, Spring supported meta-annotation attribute
overrides in custom composed annotations with reflection-based
annotation processing but not with ASM-based annotation processing.

This commit ensures that meta-annotation attribute overrides are
supported in AnnotationMetadataReadingVisitor.getAnnotationAttributes().

Issue: SPR-11574
Upgraded embedded ASM version to 5.0

Issue: SPR-11239
Polish ComponentScanParserTests and clean up warnings
Update thread pool settings in STOMP/WebSocket config

The clientInboundChannel and clientOutboundChannel now use twice
the number of available processors by default to accomodate for some
degree of blocking in task execution on average.

In practice these settings still need to be configured explicitly in
applications but these should serve as better default values than
the default values in ThreadPoolTaskExecutor.

Issue: SPR-11450
Harmonize log configuration

Prior to this commit, the codebase was using a mix of log4j.xml
and log4j.properties for test-related logging configuration. This
can be an issue as log4j takes the xml variant first when looking
for a default bootstrap configuration.

In practice, some modules declaring the properties variant were
taking the xml variant configuration from another module.

The general structure of the configuration has also been
harmonized to provide a standard console output as well as an
easy way to enable trace logs for the current module.
Fix handling of required payload.

A payload that is required will now throw an appropriate exception
regardless of if a conversion is required or not.

isEmptyPayload now takes the payload instead of the message
so that both the original payload and the converted payload, if
necessary, share the same logic.

JSR-303 validation is now consistently applied.

Issue: SPR-11577
Ensure default value of @Payload required is enforced

When no @Payload is provided, it is equivalent to @Payload with default
attribute values. Since the default value of required=true, then
an argument that's not annotated is required.
Add getter for the length of a WebSocket message
Add concurrent WebSocket session decorator (temp commit)

Issue: SPR-11586
Set custom handshakeHandler for XML sockjs config

Prior to this commit, configuring a custom handshakeHandler when setting
up a stomp-endpoint with SockJS would not be taken into account:

  <websocket:stomp-endpoint path="/foo">
    <websocket:handshake-handler ref="customHandler"/>
    <websocket:sockjs/>
  </websocket:stomp-endpoint>

This commit fixes this by creating and registering a
WebsocketTransportHandler (with this handshakeHandler) as a
transportHandler override for the SockJSService.

Issue: SPR-11568
Raise MessageConversionException in @Payload resolver

If a payload is present but conversion returns null (meaning no
converter knows how to convert), raise a MessageConversionException
that provides information about the type we were trying to convert
to and the message itself whose headers (namely content-type) contain
crucial information required to debug the problem.

Issue: SPR-11577
Javadoc: @ComponentScan is subject to @Conditional as well

Issue: SPR-11579
BufferedImageHttpMessageConverter ignores empty MIME types

Issue: SPR-11581
Polish ASM-based annotation processing

- AnnotationAttributesReadingVisitor no longer processes annotations
  from the java.lang.annotation package.

- Simplified logic in AnnotationReadingVisitorUtils
  getMergedAnnotationAttributes().

Issue: SPR-11574