"This changeset is the first step to flatten mutation_reader.
Then it introduces new mutation_fragment types for partition header and end of partition.
Using those a new flat_mutation_reader is defined.
Finally it introduces converters between new flat_mutation_reader and
old mutation_reader."
* 'haaawk/flattened_mutation_reader_v12' of github.com:scylladb/seastar-dev:
Add tests for flat_mutation_reader
Introduce conversion from flat_mutation_reader to mutation_reader
Introduce conversion from mutation_reader to flat_mutation_reader
Introduce flat_mutation_reader
Extract FlattenedConsumer concept using GCC6_CONCEPT
Introduce partition_end mutation_fragment
Introduce a position for end of partition
Introduce partition_start mutation_fragment
Introduce FragmentConsumer
Introduce a position for partition start
streamed_mutation: Extract concepts using GCC6_CONCEPT macro