mutation_readers: refactor generic partition slicing logic
mutation_readers: refactor generic partition slicing logic

There are at least 1 actual and 1 potential users for it; this
change converts the existing one.

Signed-off-by: Michael Livshin <ae60993c65de9cdf2ca07b7426c976808535d555@scylladb.com>

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