Similar to a mutation_reader, but limited: it only returns whether a key
is sure not to exist in some mutation source. Non-blocking and expected
to execute fast. Corresponds to an sstable bloom filter.
To avoid ambiguity, it doesn't return a bool, instead a longer but less
ambiguous "definitely_doesnt_exists" or "maybe_exists".
Currently column_family::for_all_partitions() relies on monotonicity
of keys. Adding strict monotonicity requirement doesn't hurt
implementaitons, but makes some consumers simpler.
