sstable_mutation_reader: consolidate constructors
sstable_mutation_reader: consolidate constructors

The two remaining sstable constructor are very similar apart from the
content of the initialize lambda. Speaking of which, the two remaining
initializer lambdas can be easily merged into one too. So this patch
does just that, consolidates the two constructors one and moves
consolidates as well as extracts the initializer method into a member
method. This means we have to store the previously captured variables as
members, but this is actually a good thing: when debugging we can see
the range and slice the reader is reading, and we are not actually
paying for it either -- they were already stored, just out of sight.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla