Test meta-@Async executor qualification

Prove that Async#value is respected even when using @Async as a meta
annotation.

Issue: SPR-6847
Add convenient WebAppInitializer base classes

This commit introduces three abstract WebApplicationInitializers, to be
used in the typical setup of a Spring-based web application.

 - AbstractContextLoaderInitializer provides an abstract base class for
   registering a ContextLoaderListener.

 - AbstractDispatcherServletInitializer provides an abstract base class
   for registering a DispatcherServlet, with an optional root context.

 - AbstractAnnotationConfigDispatcherServletInitializer provides an
   abstract base class for registering a DispatcherServlet and optional
   ContextLoaderListener based on annotated (e.g. @Configuration)
   classes.

Issue: SPR-9300
Merge pull request #62 from poutsma/SPR-9300

* SPR-9300:
  Add convenient WebAppInitializer base classes
Polish scheduled task execution infrastructure

In anticipation of substantive changes required to implement "initial
delay" support in the <task:scheduled> element and @Scheduled
annotation, the following updates have been made to the components and
infrastructure supporting scheduled task execution:

 - Fix code style violations
 - Fix compiler warnings
 - Add Javadoc where missing, update to use {@code} tags, etc.
 - Organize imports to follow conventions
Support initial delay attribute for scheduled tasks

java.util.concurrent's ScheduledExecutorService and its #schedule*
methods allow for an 'initialDelay' parameter in milliseconds.
Similarly, Spring's TaskExecutor abstraction allows for a concrete
'startTime' expressed as a Date. However, Spring's <task:scheduled> XML
element and @Scheduled annotation have, to date, not allowed for an
initial delay parameter that can be propagated down to the underlying
TaskScheduler/ScheduledExecutorService.

This commit introduces initial-delay and #initialDelay attributes to
task:scheduled and @Scheduled respectively, both indicating the number
of milliseconds to wait before the first invocation of the method in
question. Specifying a delay in this fashion is only valid in
conjunction with fixed-rate and fixed-delay tasks (i.e. not with cron
or trigger tasks).

The principal changes required to support these new attributes lie in
ScheduledTaskRegistrar, which previously supported registration of
tasks in the form of a Runnable and a Long parameter indicating (in the
case of fixed-rate and fixed-delay tasks), the interval with which the
task should be executed. In order to accommodate a third (and optional)
'initialDelay' parameter, the IntervalTask class has been added as a
holder for the Runnable to be executed, the interval in which to run
it, and the optional initial delay. For symmetry, a TriggerTask and
CronTask have also been added, the latter subclassing the former. And a
'Task' class has been added as a common ancestor for all the above.

One oddity of the implementation is in the naming of the new
setters in ScheduledTaskRegistrar. Prior to this commit, the setters
were named #setFixedDelayTasks, #setFixedRateTasks, etc, each accepting
a Map<Runnable, long>. In adding new setters for each task type, each
accepting a List<IntervalTask>, List<CronTask> etc, naturally the
approach would be to use method overloading and to introduce methods
of the same name but with differing parameter types. Unfortunately
however, Spring does not support injection against overloaded methods
(due to fundamental limitations of the underlying JDK Introspector).
This is not a problem when working with the ScheduledTaskRegistrar
directly, e.g. from within a @Configuration class that implements
SchedulingConfigurer, but is a problem from the point of view of the
ScheduledTasksBeanDefinitionParser which parses the <task:scheduled>
element - here the ScheduledTaskRegistrar is treated as a Spring bean
and is thus subject to these limitations. The solution to this problem
was simply to avoid overloading altogether, thus the naming of the new
methods ending in "List", e.g. #setFixedDelayTasksList, etc. These
methods exist primarily for use by the BeanDefinitionParser and are
not really intended for use by application developers. The Javadoc for
each of the new methods makes note of this.

Issue: SPR-7022
Merge branch cbeams/SPR-7022

* SPR-7022:
  Support initial delay attribute for scheduled tasks
  Polish scheduled task execution infrastructure