make keyPrefix default to $app->id for host-shared caches like Apc, Memcache about demo

[dicrtarasov]

@samdark
lets implements it in 3.x

$app->id is the best place for Cache::keyPrefix, and the default configuration is not suitable to work because of cache intersect conflicts in multiple web-applications.

So small code for big enhancements.
Settings Cache::keyPrefix defaults to Application::id rather then empty is the good reason.

Also, this will not "cause backwards compatibility break" in existing 2.x applications code, because the key prefix is not using in program logic never. Only empty keyPrefix may cause conflicts.

[samdark]

Could be done in the default app config.

[samdark]

That would cause backwards compatibility break. Won't be done in 2.x.

[dicrtarasov]

yes! for ex. as Requres::cookieValidationKey is generated in default config when installing :)
The difference is, the Cache::prefixKey is better in human-readable variant.