* Also put entries into the cache that allow stale-while-revalidate

* Rename phpunit.xml to phpunit.xml.dist

* Add require-dev dependency on symfony/phpunit-bridge to allow for clock mocking

See https://symfony.com/doc/current/components/phpunit_bridge.html#time-sensitive-tests.

This helps to get reliable results when time computations are under test.

* Explain in the DocBlock what exactly the computed TTL means

* Add first tests for CacheEntry

* Improve the docblock explanation

* Allow symfony/phpunit-bridge 4.4 as well

... so we get compatibility even with PHPUnit 4.

* Ensure BC with PHPUnit 4

* Avoid failing tests due to deprecation notices