absl::btree_map doesn't provide a pointer stability, so incerementing
iterator in a loop that has a scheduling point can lead to segfault.
remote_partition uses stable iterator that make a lookup on every
iterator increment.

remote_partition uses stable iterator wrapper for absl::btree. However
stable iterator doesn't support deletions from the underlying btree_map.
Add a comment about that.

Reference to the absl::btree_map is not stable, so don't use a reference
that was captured before the scheduling point.