memtable: do not update iterator_reader::_last in alloc section
memtable: do not update iterator_reader::_last in alloc section

iterator_reader::_last is a part of the state that survives allocating
section retries, therefore, it should not be modified in the retryable
context.

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,duarten/scylla,duarten/scylla