Somehow, the line of code responsible for freeing flushed nodes
in `trie_writer` is missing from the implementation.
This effectively means that `trie_writer` keeps the whole index in
memory until the index writer is closed, which for many dataset
is a guaranteed OOM.
Fix that, and add some test that catches this.
Fixesscylladb/scylladb#27082Closesscylladb/scylladb#27083
(cherry picked from commit d8e299dbb2)
Closesscylladb/scylladb#27122
30a6a2c7a7