Issue #1971158 by Berdir, beejeebus, alexpott, amateescu: Add loadMultiple() and listAll() caching to (cached) config storage.

Issue #1811372 by sdboyer: Fixed FileStorage should utilize late static binding for inheritance friendliness.

Issue #1713564 by alexpott, sun: Make Config\FileStorage instantiate Yaml\Dumper and Yaml\Parser only once.

Issue #1666632 by sun, chx: Add Config::isNew() to allow code to determine whether a config object already exists.

Issue #1605324 by sun, Jose Reyero, alexpott, chx, Rob Loach | beejeebus: Configuration system cleanup and rearchitecture.

Revert "- Patch #1605324 by sun, Jose Reyero, alexpott, chx, Rob Loach, beejeebus: configuration system cleanup and rearchitecture."

This reverts commit 8df23c8a. Original commit
was missing some files. Trying again.

- Patch #1605324 by sun, Jose Reyero, alexpott, chx, Rob Loach, beejeebus: configuration system cleanup and rearchitecture.

Issue #1470824 by alexpott, sun: Fixed XML encoder can only handle a small subset of PHP arrays, so switch to YAML.

Issue #1500238 by alexpott, sun, chx: Encode/decode data via PHP serialize()/unserialize() for DatabaseStorage (storage controller specific data formats).