Currently, we maintain a global queue of tiles, ordered by access
time, from most-recent to least-recent.  This requires us to
acquire a global mutex whenever accessing/inserting a tile, in order
to maintain the queue.

The commit replaces the global tile queue with per-instance tile
queues, ordered by the same criterion, which allows us to avoid
acquiring the global cache mutex in the above cases.  The global
mutex mainly has to be acquired only during cache construction,
destruction, and trimming.

To keep trimming simple, and relatively efficient, this commit
changes the cache eviction strategy: instead of evicting the least-
recently accessed tile first, we evict the least-recently accessed
tile of the least-recently accessed cache first (which might not be
the least-recently accessed tile overall).