lsa: Guarantee invalidated references on allocating section retry
lsa: Guarantee invalidated references on allocating section retry

There is existing code (e.g. use of partition_snapshot_row_cursor in
cache_streamed_mutation) which assumes that references will be
invalidated when bad_alloc is thrown from allocating_section. That is
currently the case because on retry we will attempt memory reclamation
which will invalidate references either through compaction or
eviction. Make this guarantee explicit.

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