Closes gh-2723

Tomcat uses the strings “on” and “off” to enable and disable
compression. YAML interprets on as true and off as false, leaving
ServerProperties.Tomcat.compression configured with “true” and “false”
respectively. One solution is to use “on” rather than on and “off”
rather than off in the YAML file but users may not realise that they
need to do so.

This commit updates the connector customiser that configures compression
to map “true” to “on” and “false” to “off”.

Closes gh-2737

Closes gh-2545

* patch-1:
  Fix typo in gradle plugin documentation

Include auto-configuration support for EhCache with auto-detection of
the default `ehcache.xml` at the root of the classpath. EhCache
configuration can also be set via `spring.cache.config`.

See gh-2633

Add support for cache manager auto-configuration that is triggered when
the `EnableCaching` annotation is added to a Spring Boot application.

A new "spring.cache" set of configuration keys is also provided. The
"spring.cache.mode" allows the user to specify the cache provider that
should be auto-configured. If no explicit configuration is provided,
the environment is checked for the best suited cache implementation,
that is:

- Generic if at least one `Cache` bean is defined in the context.
- Hazelcast if either a default configuration file is present or the
  `spring.cache.config` property is set.
- JCache if one JSR-107 provider is present
- Redis if a `RedisTemplate` is defined in the context
- Guava
- Simple as a fallback option, using concurrent maps
- NoOp (that is, no cache) if the mode is set to "none"

If the provider supports it, it is possible to specify the caches
to create on startup via `spring.cache.cache-names`. If the provider
relies on a configuration file and a custom one needs to be used
`spring.cache.config` can be set to such custom resource.

If more than one JSR-107 provider is present, it is possible to force
the provider to use by setting the mode to `jcache` and specifying the
fully qualified class name of the CachingProvider to use via
`spring.cache.jcache.provider`.

See gh-2633

Closes gh-2376

SmartApplicationListener has been superseded by GenericEventListener as
of Spring Framework 4.2. It will be eventually deprecated and removed.

Migrate our event listeners to use the new contract.

Closes gh-2576

Update MailHealthIndicator to use the new testConnection() method
available as of Spring Framework 4.2

Closes gh-2666

When configured with a context path of "", Undertow 1.1.3 changes the
context path to be "/". The change [1] was made to fix UNDERTOW-350
[2].

[1] https://github.com/undertow-io/undertow/commit/3db7707b8b34095d8d2fafecf94a10d101c62320
[2] https://issues.jboss.org/browse/UNDERTOW-350

See gh-2732

Closes gh-2670

Conflicts:
	spring-boot-dependencies/pom.xml

Closes gh-2732

Closes gh-2730

Closes gh-2731

Conflicts:
	spring-boot-dependencies/pom.xml

Conflicts:
	spring-boot-dependencies/pom.xml

gh-2735

Closes gh-2733

Closes gh-2729

Closes gh-2734

Conflicts:
	spring-boot-dependencies/pom.xml

Closes gh-2726

See gh-2718

Closes gh-2718