- It turns out, ha_rocksdb::table_flags() can return
  HA_PRIMARY_KEY_IN_READ_INDEX for all kinds of tables (as its meaning
  is "if there is a PK, PK columns contribute to the secondary index
  tuple". There is no assumption that a certain PK column can be decoded
  from the secondary index.
  (Should probably be fixed in the upstream, too, but I was unable to
   construct a testcase showing this is necessary).

- Following the above, we can undo the init_with_fields() changes in
  table.cc. MyRocks calls init_with_fields() from ha_rocksdb::open()
  which sets index-only read capabilities properly.