Impossible to iterate over non-default column family updates in WriteBatch using C API.

[konstantinvilin]

WriteBatch Iterator exposed via C API does not allow column family specific operations: put_cf, delete_cf and merge_cf. This is required when the one needs to read changes for any non-default column family. Without that functionality it is impossible to iterate over changes in WAL that are relevant to custom column families.

Expected behavior

WriteBatch iterator in C API allows walking through non-default column family changes.

Actual behavior

WriteBatch iterator in C API only iterates over default column family changes.

Steps to reproduce the behavior

1. Create WriteBatch and use put_cf, merge_cf or delete_cf to apply changes to a non-default column family.
2. Use rocksdb_writebatch_iterate C API function to iterate over WriteBatch.

[konstantinvilin]

I've submitted PR with a fix:
#12718

Could someone review it, please?