cleanup, fix and improve theme resource caching

Goals:

• fix race conditions purge_all_caches() in theme designer mode
• standardise cache directory structure - revision right after type of resource
• fix potential race conditions when serving css
• improve performance by not doing full lib/setup.php init if only core_component used
• improve coding style
• prepare fro $CFG->localcachedir

To get the best performance it is recommended to use transparent proxy such as Cloudflare - these proxies eliminate the repeated execution of PHP scripts, it is possible because data does not change and we allow infinite caching.

Note: MUC is not suitable for theme caching because the performance is critical here - loading of all core libs and MUC together with DB init would be both slow and expensive, theme caching works with config.php only and only if really necessary the setup.php continues loading all libraries and inits DB.