Refactor cache configuration

[mcaskill]

Replace the antiquated settings with a modern structure based on Stash's Symfony bundle.

Objectives

1. Rename prefix to namespace for clarity.
2. Deprecate config option types — the fallback driver concept is prone to unexpected behaviors and is unnecessary given with local environment settings.
3. Deprecate container entries cache/available-drivers, cache/drivers, cache/driver — Unnecessary overhead and broken. On top of it, the way settings must be defined in the config, this was always an unmanageable cross-driver headache.
4. Provide a PSR-16-compliant variant to the cache pool facade (or vice versa)

Proposed configuration structure

{
	"cache": {
		"active": true,

		"default_namespace": "charcoal",
		"default_ttl": 604800,

		"default_pool_class": "\\Stash\\Pool",
		"default_item_class": "\\Stash\\Item",

		"pools": {
			"first": {
				"drivers": {
					"FileSystem": {
						…
					}
				}
			},
			"second": {
				"drivers": [ "Apc", "FileSystem" ],
				"in_memory": false,

				"pool_class": "\\MyApp\\Service\\Cache\\Pool",
				"item_class": "\\MyApp\\Service\\Cache\\Item",

				"namespace": "myapp",
				"default_ttl": 86400,
				"logger": "logger"
			}
		},
		"default_pool": "first",

		"facades": {
			"first": {}
		},
		"default_facade": "first",

		"drivers": {
			…
		}
	}
}

// Pools
$first  = $container['cache'];
$first  = $container['cache/default_pool'];
$first  = $container['cache/pools']['first'];
$second = $container['cache/pools']['second'];

// Facades
$first  = $container['cache/facade'];
$first  = $container['cache/default_facade'];
$first  = $container['cache/facades']['first'];

Possible future configuration advantages

{
	"charcoal": {
		"orm": {
			"metadata_cache_driver": "cache/default_pool",
			"query_cache_driver": "cache/default_pool.second",
			"result_cache_driver": "array"
		},
		"dbal": {
			"connections": {
				"default": {
					…
				}
			},
			"default_connection": "default"
		}
	}
}