Currently, any metadata passed to ConfigCache (namely implementations of ResourceInterface) is serialized to disk. When the ConfigCache is validated, the metadata is unserialized and queried through ResourceInterface::isFresh() to determine whether the caceh is fresh. That way, ResourceInterface implementations cannot interact with services, for example a database connection.This PR introduces the new concept of ResourceCheckers. Services implementing ResourceCheckerInterface can be tagged as "config_cache.resource_checker" with an optional priority.Clients that wish to use ConfigCache can then obtain an instance from the config_cache_factory service (which implements ConfigCacheFactoryInterface). The factory will take care of injecting resource checkers into the ConfigCache instance so that they can be used for cache validation.Checking cache metadata is easy for ResourceCheckers:- First, the ResourceCheckerInterface::supports() implementation is passed the metadata object in question. If the checker cannot handle the type of resource passed, support() should return false.- Otherwise, the ResourceCheckerInterface::isFresh() method will be called and given the resource as well as the timestamp at which the cache was initialized. If that method returns false, the cache is considered stale. If it returns true, the resource is considered valid and will *not* be passed to any additional checkers.BC and migration path:This PR does not (intend to) break BC but it comes with deprecations. The main reason is that ResourceInterface contains an isFresh() method that does not make sense in the general case of resources.Thus, ResourceInterface::isFresh() is marked as deprecated and shall be removed in Symfony 3.0. Resource implementations that can (or wish to) be validated in that simple manner can implement the SelfCheckingResourceInterface sub-interface that still contains (and will keep) the isFresh() method. The change should be as simple as changing the "extends" list.Apart from that, ResourceInterface will be kept as the base interface for resource implementations. Note that ResourceInterface is used in several @api interfaces and thus cannot easily be substituted.For the Symfony 2.x series, a BCResourceInterfaceChecker will be kept that performs validation through ResourceInterface::isFresh() but will trigger a deprecation warning. The remedy is to either implement a custom ResourceChecker with a priority higher than -1000; or to switch to the aforementioned SelfCheckingResourceInterface which is used at a priority of -990 (without deprecation warning).The ConfigCache and ConfigCacheFactory classes can be used as previously but do not feature checker-based cache validation.Outlook and closing remarks:This PR supersedes#7230,#15692 and works at least in parts towards the goal of#7176.The ResourceCheckerInterface, ...ConfigCache and ...ConfigCacheFactory no longer need to be aware of the "debug" flag. The different validation rules applied previously are now just a matter of ResourceChecker configuration (i. e. "no checkers" in prod). It might be possible to remove the "debug" flag from Symfony's Router and/or Translator classes in the future as well because it was only passed on to the ConfigCache there.

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