Parallel static analysis by default

[koic]

Modern PCs have multi-core. RuboCop has --parallel option, but it is not used by default. This means that there is extra core resources for static analysis because multi-core is not being used effectively.

This PR will be parallel processing by default, so RuboCop will run faster in proportion to number of cores. Options that can be parallel processing, such as --only, --except, and others have the
same default behavior.
However, there are options that cannot be parallel processing, such as auto-correction and others, so they are serial processing as before.

--parallel can be a hidden option for some users. This change is expected to reduce user's wait time for running RuboCop.

Below is an example on my development PC.

Before:

% cd path/to/rubocop
./exe/rubocop --display-time
(snip)

1293 files inspected, no offenses detected
Finished in 61.09493000002112 seconds

After:

% cd path/to/rubocop
./exe/rubocop --display-time
(snip)

1293 files inspected, no offenses detected
Finished in 10.13813699997263 seconds

If user's PC has a lot of cores, memory and RuboCop targets a lot of files to inspect, it will be effective.

One concern is that parallel processing uses more memory. For example, Ruby's MJIT is off by default as an example of concerning about memory consumption in a low memory production runtime (e.g. Heroku's minimum Dyno).
However, RuboCop is a development tool, I assume that it will be fine in most cases.

If user want to use it in a low memory production runtime, user can specify --no-parallel.

Note: JRuby and Windows are not supported based on #4537.

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Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[dvandersluis]

The failures on this and your other PRs are resolved by #9995 (and will be fixed by #9994), so it looks like you need to rebase.

[bbatsov]

I think that's a good move! The main reason why we didn't do this earlier is that for a while the parallel analysis was rough around the edges, but given the massive time savings, I think it makes for a a better default.

P.S. 10000 is the quite the magic number! 🚀 🏅