Reduce memory use build up in disk cache queue

We use both the UTIL_QUEUE_INIT_RESIZE_IF_FULL and UTIL_QUEUE_INIT_USE_MINIMUM_PRIORITY flags so that run time compile execute as fast as possible. However when a large bunch of shaders are compiled up front and make use of all cores this can cause some excessive use of memory due to the queue never getting executed and cache items building up in memory.

This series implements two solutions to counter the problem.