feature #21039 Web server bundle (fabpot)

This PR was squashed before being merged into the 3.3-dev branch (closes #21039).

Discussion
----------

Web server bundle

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #21040
| License       | MIT
| Doc PR        | n/a

Moved the `server:*` commands to a new bundle. It makes them more easily discoverable and more decoupled. Discoverability is important when not using symfony/symfony. In that case, the commands are not available unless you have the symfony/process component installed. With a dedicated bundle, installing the bundle also installs the dependency, making the whole process easier.

Usage is the same as the current commands for basic usage:

To start a web server in the foreground:

```
bin/console server:run
```

To manage a background server:

```
bin/console server:start
bin/console server:stop
bin/console server:status
```

The big difference is that port is auto-determined if something is already listening on port 8000.

Usage is **different** if you pass options:

```
bin/console server:start 127.0.0.1:8888
bin/console server:stop # no need to pass the address again
bin/console server:status # no need to pass the address again
```

That's possible as the web server now stores its address in a pid file stored in the current directory.

Commits
-------

f39b327 [WebServerBundle] switched auto-run of server:start to off by default
961d1ce [WebServerBundle] fixed server:start when already running
126f4d9 [WebServerBundle] added support for port auto-detection
6f689d6 [WebServerBundle] changed the way we keep track of the web server
585d445 [WebServerBundle] tweaked command docs
fa7ebc5 [WebServerBundle] moved most of the logic in a new class
951a1a2 [WebServerBundle] changed wording
ac1ba77 made the router configurable via env vars
48dd2b0 removed obsolete check
132902c moved server:* command to a new bundle

composer.json

@@ -73,6 +73,7 @@
         "symfony/validator": "self.version",
         "symfony/var-dumper": "self.version",
         "symfony/web-profiler-bundle": "self.version",
+        "symfony/web-server-bundle": "self.version",
         "symfony/workflow": "self.version",
         "symfony/yaml": "self.version"
     },

src/Symfony/Bundle/FrameworkBundle/CHANGELOG.md

@@ -4,6 +4,7 @@ CHANGELOG
 3.3.0
 -----
 
+ * The server:* commands and their associated router files were moved to WebServerBundle
  * Translation related services are not loaded anymore when the `framework.translator` option
    is disabled.
  * Added `GlobalVariables::getToken()`