mutation_reader: evictable_reader: add reverse read support
mutation_reader: evictable_reader: add reverse read support

Evictable reader has to be made aware of reverse reads as it checks/edits the
slice. This shouldn't require reverse awareness normally, it is only
required because we still use the half-reversed (legacy) slice format
for reversed reads. Once we switch to the native format this commit can
be reverted.

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