CM_Class_Abstract::_getConfig should be runtime-cached

There are currently lots of APC cache hits for the same class-config keys:
```
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
ClassConfig_className:SK_Site_Admin
```

Those seem not necessary.

@tomaszdurka could you look into that?
@mariansollmann @fauvel fyi