Changes for PHP 7[.1] only versions

[sandrokeil]

If we bring a new major release of the prooph components, we should consider to change some things. /cc @prolic @codeliner

1. Should we use PHP 7.1 features (void return type, ...)?
2. Rewrite and unify of the plugin system (actually different plugin registration)
3. Documentation changes #38
4. Timeframe?
5. More ideas?
6. Get rid of uuid lib, let user decide the implementation (broadway does the same)?
7. Add a maintainer repo
8. Add a prooph component skeleton for new packages with some placeholder
9. Switch from fabpot/php-cs-fixer to squizlabs/php_codesniffer because we don't need Symfony CS
10. Add benchmarks (phpbench/phpbench) to critical components to see how to improve it
11. Commit composer.lock file to test lowest, latest and locked versions
12. Announce long term support (LTS), prefer PHP cycle?

[prolic]

1. PHP 7.1, definitely yes
2. if you have ideas how to improve it, why not
3. yep
4. no idea currently, i am starting this week with PHP 7.1 refactoring
5. to be defined....