Fix unnecessary required spring-context-support dep

Prior to this commit, a project using compile time weaving upgrading to
4.1 was forced to add spring-context-support and the jcache API in order
to build. This problem is not new really: spring-aspects holds all
aspects provided by the framework and they all are evaluated when
compiling. 4.1 just happens to define a new aspect that requires extra
dependencies.

This commit uses a new annotation of AspectJ 1.8.3. When @RequiredTypes
is added on an aspect, it is evaluated only if the classes defined on the
annotation are actually present. If they are not, the aspect is disabled
and does not break the build.

Issue: SPR-12163
@Entity no longer required with spring-aspect

This commits complements 222d2b1 for another required dependency.

javax.persistence.Entity was previously required, regardless of which
aspects were actually used by the project. It is now optional and will
not break the build if it is not present. Note that the side effect is that
AnnotationDrivenStaticEntityMockingControl is disabled as well if the
JPA API is not available.

Issue: SPR-6819
Performance improvement

Use entrySet instead of keySet followed by a lookup per key as the
former is more efficient.

Issue: SPR-12363
Merge pull request #672 from RyuSangGi/SPR-12363

* SPR-12363:
  Performance improvement
Use response encoding when escaping HTML

With SPR-9293, it is now possible to HTML escape text while taking into
account the current response encoding. When using UTF-* encodings, only
XML markup significant characters are escaped, since UTF-* natively
support those characters.

This commit adds a new servlet context parameter to enable this fix by
default in a Spring MVC application:

    <context-param>
      <param-name>responseEncodedHtmlEscape</param-name>
      <param-value>true</param-value>
    </context-param>

Issue: SPR-12350, SPR-12132
Polish Javadoc for @RequestMapping
Simple broker sends notice after disconnect

Before this change the simple broker simply removed subscriptions
upon receiving a DISCONNECT message assuming it was a result of
a client STOMP WebSocket session ending.

However, if the server-side application sends a DISCONNECT to
the broker in order to terminate a session, the STOMP WebSocket
session could remain unware without any further action. This
change ensures the simple broker sends a DISCONNECT_ACK message
downstream whenever it receives a DISCONNECT.

Issue: SPR-12288
Fix SpEL handling of function reference

These changes provide more robust handling of function
reference compilation in SpEL expressions. Prior to
this change the isCompilable check was not performing
enough visibility checks on the proposed target
function, causing bytecode to be generated that
would lead to an IllegalAccessError.
The changes also bring the argument handling for
function invocation completely inline with that used
for method invocation allowing some code to be deleted.

Many new tests are also included for function
reference compilation.

Issue: SPR-12359
Resolved package tangle in spring-expression