Avoid multiple default paths to server.pid file

Fix bug (#22811) that occurs when rails server is started in daemon mode
and optional path to the `server.pid` file is omitted. Store default path
in a constant instead of evaluating it multiple time using `File.expand_path`.

The bug in detail: The server startup procedure crashes, since it tries to
open a file at `/tmp/pids/server.pid` instead of
`<path to project>/tmp/pids/server.pid`. This bug was introduced in 51211a94bd
when Rack was upgraded from version 1.x to 2.x. Since version 2.x,
Rack does not memoize the options hash [1], and as a consequence
`Rails::Server#default_options` will be evaluated multiple times.
The hash returned by `Rails::Server#default_options` holds the default path
to the `server.pid` file. The path is generated with the method
`File.expand_path`. However, the return value of this method depends on the
current working directory [2], which changes once `Process.daemon` is invoked
by `Rack::Server#daemonize_app` and the process is detached from the current
shell.

Close #22811

[1]https://git.io/vzen2
[2]http://ruby-doc.org/core-2.1.5/File.html#method-c-expand_path

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