cache: switch to gradual cache clearing

Advantages over the old algorithm consist of the following.
 * There are no glitches due to full cache clearing.
   Items are arranged into linked list ordered by time of last use.
   Only the oldest items get deleted at the clearing event.
 * Each item now keeps track of number of references.
   Referenced cache values are immune to clearing.
 * Reduced amount of total cache memory for the same performance.
 * Reduced number of memory allocations per cache item.

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