Change FlywayMigrationStrategy from a class to an interface.

Fixes gh-3217

Extract common features from CounterBuffers and GuageBuffers into
a shared superclass. The new extracted types allows the service
implementations to be simplified.

Fixes gh-3257

Update EventPublishingRunListener to implement Ordered and use a default
order of 0. This allows for other run listeners to be added either
before or after it.

Fixes gh-3305

Create a new section in the meta-data called "hints" where users can
provide hints about a given property. The most basic use case for now
is to provide a list of values that a property can have. Each value may
have a description.

This sample JSON provides a basic example for a property called `foo.mode`
that exposes 3 values: "auto", "basic" and "advanced".

```
 "hints": [
    {
      "id": "foo.mode",
      "values": [
        {
          "value": "auto",
          "description": "Some smart description."
        },
        {
          "name": "basic"
        },
        {
          "name": "advanced"
        }
      ]
    }
]
```

This information can be read by tools (such as IDE) and offer an
auto-completion with the list of values.

Closes gh-2054

If java.io.tmpdir is configured to a directory that does not exist,
calls to File.createTempFile will fail with an IOException with the
message "The system cannot find the path specified". Unfortunately,
the path the was specified is not included in the message.

Rather than trying to automatically create the directory in what may
be a misconfigured location, we now include the value of
java.io.tmpdir in our own exception's message. Hopefully this will
help users to figure out what they've done wrong.

Closes gh-3307

The maximum and minimum number of connections are no metrics so these
are not exposed. Fix the doc that stated the contrary.

Fixes gh-3319

* pr/2294:
  Add /logfile MVC actuator endpoint

Add a `/logfile` endpoint which can be used to fetch the contents of
the log file (if one is being used).

Fixes gh-2137
Closes gh-2294

* pr/2300:
  Add a SpringBootVersion class like SpringVersion

Provide a static utility method to get the Spring Boot version based on
the implementation version of SpringApplication's package. Similar to
Spring Frameworks' SpringVersion class.

Fixes gh-2300

* pr/2704:
  Add support for ANSI colored resource banners

Add AnsiPropertySource which allows named ANSI codes to be resolved
and update ResourceBanner to include it.

This commit also deprecates constants defined in AnsiElement and
replaces them with AnsiStyle, AnsiColor and AnsiBackground enums.

Closes gh-2704

* pr/2901:
  Add Freemarker `prefer-file-system-access` support

Add a `spring.freemarker.prefer-file-system-access` property and update
FreeMarkerAutoConfiguration to support it.

Fixes gh-2901

Closes gh-3308

Closes gh-3299

* pr/3297:
  Prevent restart when META-INF/maven/** changes

These files are modified by Eclipse for some reason when you change files
like Thymeleaf HTML files. `META-INF/maven/**` has been added to the
default exclusion.

Closes gh-3295
Closes gh-3297

* pr/2764:
  Add `spring.pid.fail-on-write-error` support

Update `ApplicationPidFileWriter` to support a 'fail on write error'
properties which allows the user to exit the application if the PID
file cannot be written.

This commit also deprecates `spring.pidfile` in favor of
`spring.pid.file` so that the new property can be added without overlap.

Fixes gh-2764