misc: Move cache initialization to startup script

Guarantee that cache is initialized during startup, and only once,
instead of every time a `MetadataHandler` object is instantiated.

Also, improve logic to determine `fixtures` paths.

docker/init_scripts/init

@@ -284,6 +284,7 @@ else
 	error_log "Failed to run database migrations"
 fi
 
+# Startup process requires database and cache to be already available
 run_startup
 
 # main loop