Add auto-configuration support for TaskExecutor

This commit adds support for providing a default ThreadPoolTaskExecutor
with sensible defaults. A new TaskExecutorBuilder is provided with
defaults from the `spring.task.*` namespace and can be used to create
custom instances.

If no custom `Executor` bean is present, `@EnableAsync` now uses the
auto-configure application task executor. Same goes for the async support
in Spring MVC.

Closes gh-1563

spring-boot-project/spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -160,6 +160,13 @@ content into your application. Rather, pick only the properties that you need.
 	spring.sendgrid.proxy.host= # SendGrid proxy host.
 	spring.sendgrid.proxy.port= # SendGrid proxy port.
 
+	# TASK EXECUTION  ({sc-spring-boot-autoconfigure}/task/TaskProperties.{sc-ext}[TaskProperties])
+	spring.task.pool.allow-core-thread-timeout=true # Whether core threads are allowed to time out. This enables dynamic growing and shrinking of the pool.
+	spring.task.pool.core-size=8 # Core number of threads.
+	spring.task.pool.keep-alive=60s # Time limit for which threads may remain idle before being terminated.
+	spring.task.pool.max-size= # Maximum allowed number of threads. If tasks are filling up the queue, the pool can expand up to that size to accommodate the load. Ignored if the queue is unbounded.
+	spring.task.pool.queue-capacity= # Queue capacity. A unbounded capacity does not increase the pool and therefore ignores the "max-size" parameter.
+	spring.task.thread-name-prefix=executor- # Prefix to use for the names of newly created threads.
 
 	# ----------------------------------------
 	# WEB PROPERTIES

spring-boot-project/spring-boot-docs/src/main/asciidoc/spring-boot-features.adoc

@@ -6080,6 +6080,32 @@ in a similar manner, as shown in the following example:
 
 
 
+[[boot-features-task-execution]]
+== Task Execution
+In the absence of a `TaskExecutor` bean in the context, Spring Boot auto-configures a
+`ThreadPoolTaskExecutor` with sensible defaults that can be automatically associated to
+asynchronous task execution (`@EnableAsync`) and Spring MVC asynchronous request
+processing.
+
+The thread pool uses 8 core threads that can grow and shrink according to the load. Those
+default settings can be fine-tuned using the `spring.task` namespace as shown in the
+following example:
+
+[source,properties,indent=0]
+----
+	spring.task.pool.max-threads=16
+	spring.task.pool.queue-capacity=100
+	spring.task.pool.keep-alive=10s
+----
+
+This changes the thread pool to use a bounded queue so that when the queue is full (100
+tasks), the thread pool increases to maximum 16 threads. Shrinking of the pool is more
+aggressive as well as threads are reclaimed when they are idle for 10 seconds (rather than
+60 seconds by default).
+
+
+
+
 [[boot-features-integration]]
 == Spring Integration
 Spring Boot offers several conveniences for working with {spring-integration}[Spring