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From Pawel: This series enables row cache to serve range queries. In order to achieve that row cache needs to know whether there are some other partitions in the specified range that are not cached and need to be read from the sstables. That information is provied by key_readers, which work very similarly to mutation_readers, but return only the decorated key of partitions in range. In case of sstables key_readers is implemented to use partition index. Approach like this has the disadvantage of needing to access the disk even if all partitions in the range are cached. There are (at least) two solutions ways of dealing with that problem: - cache partition index - that will also help in all other places where it is neededed - add a flag to cache_entry which, when set, indicates that the immediate successor of the partition is also in the cache. Such flag would be set by mutation reader and cleared during eviction. It will also allow newly created mutations from memtable to be moved to cache provided that both their successors and predecessors are already there. The key_reader part of this patchsets adds a lot of new code that probably won't be used in any other place, but the alternative would be to always interleave reads from cache with reads from sstables and that would be more heavy on partition index, which isn't cached. Fixes #185.