Limit total cache size

[nschoellhorn]

The cache currently doesn't care what the capacity setting is. We should implement limiting the total cached amount by adding eviction of older items that weren't accessed for a longer time.
The database model already supports that, offering a last_read and last_write column to mark when chunks were last accessed. Those values need to be updated in read() and write() and then checked when initializing new chunks. When the new chunk would exceed the allowed size, it needs to find the chunk with the oldest last_read and last_write times and evict that chunk from the cache.