In case of SELECT DISTINCT statments we are not intersted in clustering
keys at all. The only important information is whether partition key
exists and what's in static row (if it exists).
Signed-off-by: Paweł Dziepak <pdziepak@cloudius-systems.com>
Mutation query differs from data query in that returns information
needed to reconcile data slice with that retruned by other data
sources.
There is a generic mutation_query() algorithm introduced, which can
work with any mutation_source.
database::query_mutations() is a shard-local interface for mutation
queries.
The reconcilable_result is introduced as a medium for mutation query
results. It piggy backs on frozen_mutation as a medium for
reconcilable data.
