engine attributes in partitioning is broken

For not partitioned tables, engines currently read options from table->s->option_struct.
This doesn't work, if different partitions can have different options, because they share the same TABLE. For this reason option_struct is also stored per partition and per subpartition in partition_element. For engines to be able to find options without detecting what partition or subpartition it belongs to (some do that!), ha_partition does

  tbl->s->connect_string= part_elem->connect_string;

  if (part_elem->option_list)

    tbl->s->option_list= part_elem->option_list;

  if (part_elem->option_struct)

    tbl->s->option_struct= part_elem->option_struct;

before invoking handler::ha_create of an individual partition.

This obviously can only work during creation, but an engine can technically consult its option_struct any time and when a TABLE_SHARE is used concurrently by many threads it cannot have its TABLE_SHARE::option_struct pointing to some individual partition. Same problem with the connect_string.

Let's

• rename TABLE_SHARE::option_struct to option_struct_table to emphasize that it's options for the whole table (and catch incorrect usages of it)
• rename partition_element::option_struct to option_struct_part, ditto
• introduce handler::option_struct which will point to option_struct_table or option_struct_part as appropriate, the handler can use it any time as needed. A handler should not use option_struct_table unless it really knows what it's doing and has verified that it doesn't break partitioning.
• move connect_string into option_struct