TABLE_SHARE::init_from_binary_frm_image() calls handler_file->index_flags()
before it has set TABLE_SHARE::primary_key (it is 0 while it should be
MAX_KEY in my example).
This causes MyRocks to report wrong index flags (it thinks it's a PK while
it is not), which causes invalid query plans later on.

Do the only thing that seems feasible: adjust field->part_of key to have
correct value in ha_rocksdb::open.