Move stuff contained therein to `sstable_mutation_reader.{hh,cc}` which
will serve as the collection point of utility stuff needed by all reader
implementations.
Move all the mx format specific context and consumer code to
mx/reader.cc and add a factory function `mx::make_reader()` which takes
over the job of instantiating the `sstable_mutation_reader` with the mx
specific context and consumer.